Identified Intangible Assets and Liabilities

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



(Amounts in thousands)	Balance as of December 31,
	2018			2017
Identified intangible assets:
Gross amount	$	308,895		$	310,097
Accumulated amortization	(172,114		)	(150,837		)
Total, net	$	136,781		$	159,260
Identified intangible liabilities (included in deferred revenue):
Gross amount	$	503,373		$	530,497
Accumulated amortization	(341,779		)	(324,897		)
Total, net	$	161,594		$	205,600

Amortization of acquired below-market leases, net of acquired above-market leases, resulted in an increase to rental income of $38,573,000, $46,103,000 and $51,849,000 for the years ended December 31, 2018, 2017 and 2016, 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	$	24,661
2020	23,591
2021	18,857
2022	15,746
2023	13,215

Amortization of all other identified intangible assets (a component of depreciation and amortization expense) was $18,018,000, $25,057,000 and $28,897,000 for the years ended December 31, 2018, 2017 and 2016, 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	$	13,726
2020	13,513
2021	11,974
2022	10,244
2023	10,157

We are a tenant under ground leases at 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 $1,747,000 for each of the years ended December 31, 2018, 2017 and 2016, 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