The following summarizes our identified intangible assets (primarily acquired above-market leases) and liabilities (primarily acquired below-market leases) as of December 31, 2015 and 2014.

(Amounts in thousands)	Balance as of December 31,
	2015		2014
Identified intangible assets:
Gross amount	$	415,261	$	424,976
Accumulated amortization		(187,360)		(199,821)
Net	$	227,901	$	225,155
Identified intangible liabilities (included in deferred revenue):
Gross amount	$	643,488	$	657,976
Accumulated amortization		(325,340)		(329,775)
Net	$	318,148	$	328,201

Amortization of acquired below-market leases, net of acquired above-market leases, resulted in an increase to rental income of $78,749,000, $37,516,000 and $41,970,000 for the years ended December 31, 2015, 2014 and 2013, 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, 2016 is as follows:

(Amounts in thousands)
2016	$	52,359
2017		44,501
2018		43,028
2019		31,011
2020		23,320

Amortization of all other identified intangible assets (a component of depreciation and amortization expense) was $36,659,000, $28,275,000 and $61,915,000 for the years ended December 31, 2015, 2014 and 2013, 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, 2016 is as follows:

(Amounts in thousands)
2016	$	29,349
2017		24,427
2018		20,063
2019		15,779
2020		12,345

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 of $1,832,000, $1,832,000, and $2,745,000 for the years ended December 31, 2015, 2014 and 2013. Estimated annual amortization of these below-market leases, net of above-market leases, for each of the five succeeding years commencing January 1, 2016 is as follows:

(Amounts in thousands)
2016	$	1,832
2017		1,832
2018		1,832
2019		1,832
2020		1,832