8.Intangible Assets and Liabilities

The following table summarizes our intangible assets (acquired above-market leases and acquired in-place leases) and intangible liabilities (acquired below-market leases) as of December 31, 2017 and 2016.

	As of December 31,
(Amounts in thousands)	2017			2016
Intangible assets:
Gross amount	$	553,063		$	579,066
Accumulated amortization		(200,857	)		(166,841	)
	$	352,206		$	412,225
Intangible liabilities:
Gross amount	$	205,101		$	208,367
Accumulated amortization		(75,073	)		(55,349	)
	$	130,028		$	153,018

Amortization of acquired below-market leases, net of acquired above-market leases, resulted in an increase to “rental income” of $19,523,000, $9,536,000 and $9,917,000 for the years ended December 31, 2017, 2016 and 2015, 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, 2018 is as follows.

(Amounts in thousands)
2018	$	16,803
2019		13,965
2020		8,660
2021		4,404
2022		1,006

Amortization of acquired in-place leases (a component of “depreciation and amortization” expense) was $76,016,000, $94,935,000 and $128,603,000 for the years ended December 31, 2017, 2016 and 2015, respectively. Estimated annual amortization of acquired in-place leases for each of the five succeeding years commencing January 1, 2018 is as follows.

(Amounts in thousands)
2018	$	59,880
2019		54,122
2020		42,347
2021		30,159
2022		24,049