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 June 30, 2016 and December 31, 2015.

	As of
(Amounts in thousands)	June 30, 2016			December 31, 2015
Intangible assets:
Gross amount	$	592,133		$	655,194
Accumulated amortization		(156,683	)		(143,987	)
	$	435,450		$	511,207
Intangible liabilities:
Gross amount	$	217,021		$	221,672
Accumulated amortization		(62,363	)		(41,931	)
	$	154,658		$	179,741

Amortization of acquired below-market leases, net of acquired above-market leases, resulted in an increase to rental income of $7,100,000 and $872,000 for the three months ended June 30, 2016 and 2015, respectively, and $3,481,000 and $1,762,000 for the six months ended June 30, 2016 and 2015, respectively. The three and six months ended June 30, 2016 includes $3,915,000 and $7,830,000 of income, respectively, from the accelerated amortization of a below-market lease liability in connection with a tenant’s lease modification. The six months ended June 30, 2016 also includes a $9,834,000 of write-off of a tenant’s above-market lease asset in connection with a lease termination. 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 $23,463,000 and $37,638,000 for the three months ended June 30, 2016 and 2015, respectively and $54,155,000 and $70,630,000 for the six months ended June 30, 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