14. SEGMENT FINANCIAL INFORMATION Our four reportable segments are office, retail, multi-family and industrial. Factors used to determine our reportable segments include the physical and economic characteristics of our properties and the related operating activities. Our chief operating decision makers rely on net operating income, among other factors, to make decisions about allocating resources and assessing segment performance. Net operating income is the key performance metric that captures the unique operating characteristics of each segment. Items that are not directly assignable to a segment, such as certain corporate items, are not allocated but reflected as reconciling items. The following table sets forth the financial results by segment for the years ended December 31, 2019 , 2018 and 2017 : (in thousands) Office Retail Multi-family Industrial Corporate Consolidated 2019 Rental revenues $ 93,826 $ 70,462 $ 6,418 $ 13,735 $  $ 184,441 Rental expenses (37,905 ) (17,357 ) (2,864 ) (2,934 )  (61,060 ) Net operating income (loss) $ 55,921 $ 53,105 $ 3,554 $ 10,801 $  $ 123,381 Real estate-related depreciation and amortization $ 26,194 $ 20,317 $ 4,028 $ 6,803 $  $ 57,342 Total assets $ 458,583 $ 652,707 $ 293,498 $ 207,844 $ 165,633 $ 1,778,265 2018 Rental revenues $ 108,421 $ 73,416 $  $ 7,794 $  $ 189,631 Rental expenses (42,544 ) (17,618 )  (1,505 )  (61,667 ) Net operating income $ 65,877 $ 55,798 $  $ 6,289 $  $ 127,964 Real estate-related depreciation and amortization $ 33,335 $ 20,616 $  $ 3,915 $  $ 57,866 Total assets $ 724,875 $ 671,007 $  $ 111,230 $ 73,990 $ 1,581,102 2017 Rental revenues $ 108,305 $ 81,871 $  $ 6,342 $  $ 196,518 Rental expenses (44,520 ) (20,388 )  (1,624 )  (66,532 ) Net operating income $ 63,785 $ 61,483 $  $ 4,718 $  $ 129,986 Real estate-related depreciation and amortization $ 41,283 $ 24,216 $  $ 2,571 $  $ 68,070 Total assets $ 775,917 $ 705,696 $  $ 58,657 $ 67,836 $ 1,608,106 We consider net operating income to be an appropriate supplemental performance measure and believe net operating income provides useful information to our investors regarding our financial condition and results of operations because net operating income reflects the operating performance of our properties and excludes certain items that are not considered to be controllable in connection with the management of the properties, such as real estate-related depreciation and amortization, general and administrative expenses, advisory fees, impairment charges, interest expense, gains on sale of properties, other income and expense, gains and losses on the extinguishment of debt and noncontrolling interests. However, net operating income should not be viewed as an alternative measure of our financial performance since it excludes such items, which could materially impact our results of operations. Further, our net operating income may not be comparable to that of other real estate companies, as they may use different methodologies for calculating net operating income. Therefore, we believe net income, as defined by GAAP, to be the most appropriate measure to evaluate our overall financial performance. The following table is a reconciliation of our reported net income attributable to common stockholders to our net operating income for the years ended December 31, 2019 , 2018 and 2017 . For the Year Ended December 31, (in thousands) 2019 2018 2017 Net income (loss) attributable to common stockholders $ 142,551 $ (1,237 ) $ 72,216 Debt-related income (227 ) (694 ) (828 ) Real estate-related depreciation and amortization 57,342 57,866 68,070 General and administrative expenses 8,985 8,817 9,235 Advisory fees, related party 17,413 14,149 13,285 Impairment of real estate property 113 14,648 1,116 Other (income) expense (153 ) 251 462 Interest expense 48,170 48,358 42,305 Gain on sale of real estate property (160,537 ) (14,093 ) (83,057 ) Gain on extinguishment of debt and financing commitments, net (1,002 )   Net income (loss) attributable to noncontrolling interests 10,726 (101 ) 7,182 Net operating income $ 123,381 $ 127,964 $ 129,986