5. AMORTIZING INTANGIBLE ASSETS AND UNFAVORABLE GAS GATHERING CONTRACT

Details regarding our intangible assets and the unfavorable gas gathering contract (included in other noncurrent liabilities), all of which are subject to amortization, follow.

	December 31, 2017
	Gross carrying amount		Accumulated amortization			Net
	(In thousands)
Favorable gas gathering contracts	$	24,195	$	(12,350	)	$	11,845
Contract intangibles		278,448		(117,821	)		160,627
Rights-of-way		159,986		(31,113	)		128,873
Total intangible assets	$	462,629	$	(161,284	)	$	301,345

Unfavorable gas gathering contract	$	10,962	$	(9,074	)	$	1,888

	December 31, 2016
	Gross carrying amount		Accumulated amortization			Net
	(In thousands)
Favorable gas gathering contracts	$	24,195	$	(10,795	)	$	13,400
Contract intangibles		426,464		(146,468	)		279,996
Rights-of-way		153,015		(24,959	)		128,056
Total intangible assets	$	603,674	$	(182,222	)	$	421,452

Unfavorable gas gathering contract	$	10,962	$	(6,916	)	$	4,046

In December 2017, in connection with certain strategic initiatives, we evaluated certain long-lived assets relating to the Bison Midstream system within the Williston Basin reporting segment (see Note 4). In connection with this evaluation, we evaluated the related intangible assets associated therewith for impairment consisting of contract intangible assets and rights-of-way intangible assets. We concluded the contract intangible assets were also impaired and, as a result, we recorded an impairment charge of $85.2 million.

During the fourth quarter of 2015, we identified a need to evaluate the goodwill associated with certain of our gathering systems (see Note 6). In connection with these evaluations, we also evaluated the related intangible assets associated therewith for impairment and concluded that no impairment was necessary.

We recognized amortization expense in other revenues as follows:

	Year ended December 31,
	2017			2016			2015
	(In thousands)
Amortization expense – favorable gas gathering contracts	$	(1,555	)	$	(1,261	)	$	(1,478	)
Amortization expense – unfavorable gas gathering contract		2,158			839			692

We recognized amortization expense in costs and expenses as follows:

	Year ended December 31,
	2017		2016		2015
	(In thousands)
Amortization expense – contract intangibles	$	34,202	$	35,416	$	35,339
Amortization expense – rights-of-way		6,153		6,053		5,863

The estimated aggregate annual amortization expected to be recognized for as of December 31, 2017 for each of the five succeeding fiscal years follows.

	Intangible assets		Unfavorable gas gathering contract
	(In thousands)
2018	$	34,026	$	1,888
2019		33,139		—
2020		32,963		—
2021		29,271		—
2022		26,204		—