Identified Intangible Assets and Liabilities

The following summarizes our identified intangible assets (primarily above-market leases) and liabilities (primarily acquired below-market leases) as of March 31, 2018 and December 31, 2017.



(Amounts in thousands)	Balance as of
	March 31, 2018			December 31, 2017
Identified intangible assets:
Gross amount	$	309,896		$	310,097
Accumulated amortization	(157,062		)	(150,837		)
Total, net	$	152,834		$	159,260
Identified intangible liabilities (included in deferred revenue):
Gross amount	$	529,951		$	530,497
Accumulated amortization	(336,049		)	(324,897		)
Total, net	$	193,902		$	205,600

Amortization of acquired below-market leases, net of acquired above-market leases, resulted in an increase to rental income of $10,581,000 and $11,116,000 for the three months ended March 31, 2018 and 2017, respectively. Estimated annual amortization of acquired below-market leases, net of acquired above-market leases, for each of the five succeeding years commencing January 1, 2019 is as follows:



(Amounts in thousands)
2019	$	30,529
2020	22,246
2021	17,475
2022	14,292
2023	11,590

Amortization of all other identified intangible assets (a component of depreciation and amortization expense) was $4,876,000 and $6,981,000 for the three months ended March 31, 2018 and 2017, respectively. Estimated annual amortization of all other identified intangible assets including acquired in-place leases, customer relationships, and third party contracts for each of the five succeeding years commencing January 1, 2019 is as follows:



(Amounts in thousands)
2019	$	15,169
2020	11,961
2021	10,981
2022	9,426
2023	9,295

We are a tenant under ground leases for certain properties. Amortization of these acquired below-market leases, net of above-market leases, resulted in an increase to rent expense (a component of operating expense) of $437,000 for the three months ended March 31, 2018 and 2017, respectively. Estimated annual amortization of these below-market leases, net of above-market leases, for each of the five succeeding years commencing January 1, 2019 is as follows:



(Amounts in thousands)
2019	$	1,747
2020	1,747
2021	1,747
2022	1,747
2023	1,747