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 September 30, 2018 and December 31, 2017.



(Amounts in thousands)	Balance as of
	September 30, 2018			December 31, 2017
Identified intangible assets:
Gross amount	$	307,855		$	310,097
Accumulated amortization	(167,861		)	(150,837		)
Total, net	$	139,994		$	159,260
Identified intangible liabilities (included in deferred revenue):
Gross amount	$	508,468		$	530,497
Accumulated amortization	(338,665		)	(324,897		)
Total, net	$	169,803		$	205,600

Amortization of acquired below-market leases, net of acquired above-market leases, resulted in an increase to rental income of $10,373,000 and $11,054,000 for the three months ended September 30, 2018 and 2017, respectively, and $31,480,000 and $34,758,000 for the nine months ended September 30, 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	$	25,115
2020	24,047
2021	19,313
2022	16,173
2023	13,496

Amortization of all other identified intangible assets (a component of depreciation and amortization expense) was $4,822,000 and $6,069,000 for the three months ended September 30, 2018 and 2017, respectively, and $14,557,000 and $19,896,000 for the nine months ended September 30, 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	$	12,902
2020	12,817
2021	11,838
2022	10,286
2023	10,158

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 and $437,000 for the three months ended September 30, 2018, and 2017, respectively, and $1,310,000 and $1,310,000 for the nine months ended September 30, 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