Business Segment Information - Reconciliation of Segment Profit to Income Before Taxes (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of segment profit to income before income taxes
Income before income taxes	$ 50,386	$ 44,975	$ 10,588	$ 29,275	$ 62,246	$ 43,940	$ 13,927	$ 25,404	$ 135,224	$ 145,517	$ 71,116
Stock compensation expense	7,282	10,323	11,371	10,140	9,849	9,176	9,510	8,887	39,116	37,422	50,384
Changes in fair value of contingent consideration	1,327	(834)	252	(49)	(614)	313	463	(266)	696	(104)	44,257
Impairment of intangible assets							4,800			4,800
Non-controlling interest segment (profit) loss									59	737	2,634
Depreciation and amortization	32,810	28,189	27,731	26,976	28,574	26,885	25,580	25,007	115,706	106,046	102,844
Interest expense	9,266	7,663	4,900	4,148	3,413	3,038	1,994	1,748	25,977	10,193	6,581
Interest and other income	$ (2,086)	$ (1,781)	$ (1,071)	$ (949)	$ (702)	$ (741)	$ (692)	$ (683)	(5,887)	(2,818)	(2,165)
Segment profit (loss)									$ 310,891	$ 301,793	$ 275,651