7. Intangible Assets and Liabilities

The following is a summary of intangible assets and liabilities and related accumulated amortization at December 31:

(in thousands)	December 31,
	2017			2016
Lease intangibles:
Acquired above-market leases	$	59,502		$	45,490
Less accumulated amortization		(9,183	)		(4,940	)
Acquired above-market leases, net		50,319			40,550
Acquired in-place leases		216,858			141,676
Less accumulated amortization		(24,518	)		(14,105	)
Acquired in-place leases, net		192,340			127,571
Total intangible lease assets, net	$	242,659		$	168,121
Acquired below-market leases	$	91,667		$	54,062
Less accumulated amortization		(9,923	)		(6,191	)
Intangible lease liabilities, net	$	81,744		$	47,871
Leasing fees	$	16,286		$	13,279
Less accumulated amortization		(2,732	)		(1,950	)
Leasing fees, net	$	13,554		$	11,329

Amortization expense was $11,903, $8,345 and $4,349 for acquired in-place leases and leasing fees for the years ended December 31, 2017, 2016, and 2015, respectively. Amortization of acquired above-market and below-market leases, net, was a (decrease) increase in rental income of $(496), $482 and $569 for the years ended December 31, 2017, 2016, and 2015, respectively.

Estimated future amortization of intangible assets and liabilities for at December 31, 2017 is as follows:

(in thousands)
2018	$	16,009
2019		13,452
2020		13,321
2021		13,282
2022		13,136
Thereafter		105,269
	$	174,469