Real Estate Intangibles

3 Months Ended

Mar. 31, 2018

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):

		March 31, 2018		December 31, 2017
Assets:
	In place lease intangibles	$	1,395,920	$	1,352,139
	Above market tenant leases		58,402		58,443
	Below market ground leases		59,030		58,784
	Lease commissions		35,269		33,105
	Gross historical cost		1,548,621		1,502,471
	Accumulated amortization		(1,149,223)		(1,125,437)
	Net book value	$	399,398	$	377,034

	Weighted-average amortization period in years		14.4		15.1

Liabilities:
	Below market tenant leases	$	71,706	$	60,430
	Above market ground leases		8,540		8,540
	Gross historical cost		80,246		68,970
	Accumulated amortization		(40,400)		(39,629)
	Net book value	$	39,846	$	29,341

	Weighted-average amortization period in years		16.0		20.1

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


	Three Months Ended
	March 31,
	2018		2017
Rental income related to above/below market tenant leases, net	$	(351)	$	304
Property operating expenses related to above/below market ground leases, net		367		(312)
Depreciation and amortization related to in place lease intangibles and lease commissions		(32,261)		(39,302)

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

		Assets		Liabilities
2018	$	94,521	$	4,470
2019		74,771		5,582
2020		47,871		5,085
2021		20,985		4,597
2022		17,770		4,132
Thereafter		143,480		15,980
Total	$	399,398	$	39,846