(6) Goodwill and Intangible Assets

Goodwill

Changes in the carrying amount of goodwill were as follows:

 

Balance at December 31, 2011		$	90,008
Additions—Power Fuels merger			312,715
Additions—Other acquisitions			12,453
Balance at December 31, 2012			415,176
Additions—2013 acquisitions			341
Acquisition adjustments, net			(6,821	)
Balance at December 31, 2013		$	408,696

During 2013, the Company recorded net adjustments to goodwill for the settlement of the working capital adjustment for Power Fuels and the finalization of tangible asset valuations for Power Fuels.

 

Intangible Assets

Intangible assets consist of the following:

 

		December 31, 2013																December 31, 2012
		Gross Carrying Amount				Accumulated Amortization				Net				Remaining Useful Life				Gross Carrying Amount				Accumulated Amortization				Net				Remaining Useful Life
Customer relationships		$	163,106			$	(28,127	)		$	134,979				13.6			$	166,426			$	(8,945	)		$	157,481				14.3
Disposal permits			1,819				(682	)			1,137				7.2				2,588				(472	)			2,116				8.2
Customer contracts			17,352				(4,105	)			13,247				13.0				17,352				(3,073	)			14,279				14.0
		$	182,277			$	(32,914	)		$	149,363				13.4			$	186,366			$	(12,490	)		$	173,876				14.2

The remaining weighted average useful lives shown are calculated based on the net book value and remaining amortization period of each respective intangible asset. During the years ended December 31, 2013 and 2012, the Company recorded impairment charges of $4.5 million and $2.4 million, respectively, for the write-down of certain intangible assets (Note 7).

Amortization expense was $20.4 million, $7.5 million and $3.9 million for the years ended December 31, 2013, 2012 and 2011, respectively. As of December 31, 2013, future amortization expense of intangible assets is estimated to be:

 

2014		$	16,681
2015			15,643
2016			13,930
2017			13,310
2018			12,889
Thereafter			76,910
Total		$	149,363