Business Segment Information - Reconciliation of Segment Profit to Income Before Taxes (Details) - USD ($) $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of segment profit to income before income taxes
Income before income taxes	$ (27,519)	$ 40,961	$ 18,375	$ 11,377	$ 50,386	$ 44,975	$ 10,588	$ 29,275		$ 43,194	$ 135,224	$ 145,517
Stock compensation expense	2,067	9,320	10,439	7,646	7,282	10,323	11,371	10,140		29,472	39,116	37,422
Changes in fair value of contingent consideration	856	148	70	233	1,327	(834)	252	(49)		1,307	696	(104)
Impairment of intangible assets									$ 4,800			4,800
Non-controlling interest segment (profit) loss											59	737
Depreciation and amortization	35,358	33,047	33,848	30,407	32,810	28,189	27,731	26,976		132,660	115,706	106,046
Interest expense	9,362	8,990	8,678	8,366	9,266	7,663	4,900	4,148		35,396	25,977	10,193
Interest and other income	$ (4,090)	$ (4,139)	$ (3,363)	$ (2,476)	$ (2,086)	$ (1,781)	$ (1,071)	$ (949)		(14,068)	(5,887)	(2,818)
Segment profit (loss)										$ 227,961	$ 310,891	$ 301,793