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 June 30, 2018 and December 31, 2017.



(Amounts in thousands)	Balance as of
	June 30, 2018			December 31, 2017
Identified intangible assets:
Gross amount	$	309,776		$	310,097
Accumulated amortization	(163,406		)	(150,837		)
Total, net	$	146,370		$	159,260
Identified intangible liabilities (included in deferred revenue):
Gross amount	$	527,766		$	530,497
Accumulated amortization	(346,474		)	(324,897		)
Total, net	$	181,292		$	205,600

Amortization of acquired below-market leases, net of acquired above-market leases, resulted in an increase to rental income of $10,526,000 and $12,588,000 for the three months ended June 30, 2018 and 2017, respectively, and $21,107,000 and $23,704,000 for the six months ended June 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	$	29,871
2020	22,213
2021	17,478
2022	14,339
2023	11,661

Amortization of all other identified intangible assets (a component of depreciation and amortization expense) was $4,876,000 and $6,846,000 for the three months ended June 30, 2018 and 2017, respectively, and $9,735,000 and $13,827,000 for the six months ended June 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	$	14,994
2020	12,008
2021	11,030
2022	9,477
2023	9,349

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 June 30, 2018, and 2017, respectively, and $874,000 and $874,000 for the six months ended June 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