Real Estate Intangibles

12 Months Ended

Dec. 31, 2015

4. Real Estate Intangibles

The following is a summary of our real estate intangibles, excluding those classified as held for sale, as of the dates indicated (dollars in thousands):


		December 31, 2015		December 31, 2014
Assets:
	In place lease intangibles	$	1,179,537	$	988,290
	Above market tenant leases		67,529		65,684
	Below market ground leases		80,224		62,426
	Lease commissions		23,295		19,536
	Gross historical cost		1,350,585		1,135,936
	Accumulated amortization		(881,096)		(776,501)
	Net book value	$	469,489	$	359,435

	Weighted-average amortization period in years		13.4		17.7

Liabilities:
	Below market tenant leases	$	93,089	$	91,168
	Above market ground leases		7,907		7,859
	Gross historical cost		100,996		99,027
	Accumulated amortization		(46,048)		(40,891)
	Net book value	$	54,948	$	58,136

	Weighted-average amortization period in years		14.5		14.4

The following is a summary of real estate intangible amortization for the periods presented (in thousands):


	Year Ended December 31,
	2015		2014		2013
Rental income related to above/below market tenant leases, net	$	(2,746)	$	509	$	748
Property operating expenses related to above/below market ground leases, net		(1,272)		(1,248)		(1,208)
Depreciation and amortization related to in place lease intangibles and lease commissions		(115,855)		(214,966)		(246,938)

The future estimated aggregate amortization of intangible assets and liabilities is as follows for the periods presented (in thousands):


		Assets		Liabilities
2016	$	137,635	$	7,523
2017		81,166		6,812
2018		47,283		6,185
2019		25,582		5,775
2020		22,163		5,294
Thereafter		155,660		23,359
Totals	$	469,489	$	54,948