6.Intangible Assets and Liabilities

The following summarizes our intangible assets (primarily acquired above-market leases and acquired in-place leases) and intangible liabilities (primarily acquired below-market leases) as of March 31, 2016 and December 31, 2015.

	As of
(Amounts in thousands)	March 31, 2016			December 31, 2015
Intangible assets:
Gross amount	$	621,466		$	655,194
Accumulated amortization		(157,239	)		(143,987	)
	$	464,227		$	511,207
Intangible liabilities:
Gross amount	$	221,052		$	221,672
Accumulated amortization		(53,983	)		(41,931	)
	$	167,069		$	179,741

Amortization of acquired below-market leases, net of acquired above-market leases, resulted in a $3,619,000 decrease to rental income for the three months ended March 31, 2016 (primarily due to a $9,834,000 write-off of a tenant’s above market lease asset in connection with its lease termination) and an $890,000 increase to rental income for the three months ended March 31, 2015. Estimated annual amortization of acquired below-market leases, net of acquired above-market leases, for each of the five succeeding years commencing January 1, 2017 is as follows.

(Amounts in thousands)
2017	$	7,892
2018		9,800
2019		9,118
2020		7,744
2021		3,548

Amortization of acquired in-place leases (a component of depreciation and amortization expense) was $30,692,000 and $32,992,000 for the three months ended March 31, 2016 and 2015, respectively. Estimated annual amortization of acquired in-place leases for each of the five succeeding years commencing January 1, 2017 is as follows.

(Amounts in thousands)
2017	$	55,004
2018		48,657
2019		44,045
2020		38,531
2021		26,355