Revenue Recognition We record rental revenues on a straight-line basis under which contractual rent increases are recognized evenly over the lease term. Certain properties have leases that provide for tenant occupancy during periods where no rent is due or where minimum rent payments change during the term of the lease. Accordingly, we record receivables from tenants that we expect to collect over the remaining lease term rather than currently, which are recorded as a straight-line rent receivable. When we acquire a property, the terms of existing leases are considered to commence as of the acquisition date for the purposes of this calculation. The total increase to  Rental revenues  due to straight-line rent adjustments was approximately $ 2.1 million and $1.4 million, for the three months ended March 31, 2014 and 2013, respectively. Tenant recovery income includes payments and amounts due from tenants pursuant to their leases for real estate taxes, insurance and other recoverable property operating expenses and is recognized as  Rental revenues  during the same period the related expenses are incurred. Tenant recovery income recognized as  Rental revenues  was approximately $ 20.0 million and $ 15.2 million, respectively, for the three months ended March 31, 2014 and 2013, respectively. We maintain an allowance for estimated losses that may result from the inability of our tenants to make required payments. If a tenant fails to make contractual payments beyond any allowance, we may recognize additional bad debt expense in future periods equal to the net outstanding balances. In connection with property acquisitions qualifying as business combinations, we may acquire leases with rental rates above or below the market rental rates. Such differences are recorded as an intangible lease asset or liability and amortized to  Rental revenues  over the reasonably assured term of the related leases. The unamortized balances of these assets and liabilities associated with the early termination of leases are fully amortize