The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

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Goodwill and Intangible Assets - Intangible Assets (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Intangible assets
Weighted Average Life			15 years
Gross Carrying Amount	$ 132,565		$ 132,565		$ 84,250
Accumulated Amortization	(46,752)		(46,752)		(39,450)
Amortization expense of intangible assets	3,100	$ 2,600	8,287	$ 6,184
Estimated future amortization expense for intangible assets
2018 (remaining six months)	3,015		3,015
2019	11,879		11,879
2020	9,134		9,134
2021	7,897		7,897
2022	6,736		6,736
Thereafter	47,152		47,152
Total	85,813		$ 85,813		$ 44,800
Tradename
Intangible assets
Weighted Average Life			9 years		9 years
Gross Carrying Amount	31,390		$ 31,390		$ 32,175
Accumulated Amortization	(24,272)		$ (24,272)		$ (22,238)
Customer relationships
Intangible assets
Weighted Average Life			16 years		16 years
Gross Carrying Amount	97,400		$ 97,400		$ 49,900
Accumulated Amortization	(21,146)		$ (21,146)		$ (16,338)
Non-compete agreements
Intangible assets
Weighted Average Life			5 years		5 years
Gross Carrying Amount	3,500		$ 3,500		$ 1,900
Accumulated Amortization	(1,212)		$ (1,212)		$ (820)
Other
Intangible assets
Weighted Average Life			3 years		3 years
Gross Carrying Amount	275		$ 275		$ 275
Accumulated Amortization	$ (122)		$ (122)		$ (54)