Business Segment Information - Reconciliation of Segment Profit to Income Before Taxes (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reconciliation of segment profit to income before income taxes
Income before income taxes	$ 62,246	$ 43,940	$ 13,927	$ 25,404	$ 64,524	$ (15,015)	$ 10,280	$ 11,327	$ 145,517	$ 71,116	$ 117,920
Stock compensation expense	9,849	9,176	9,510	8,887	9,791	12,897	13,795	13,901	37,422	50,384	40,584
Changes in fair value of contingent consideration	(614)	313	463	(266)	(3,017)	29,738	2,567	14,969	(104)	44,257	6,172
Impairment of intangible assets			4,800						4,800
Non-controlling interest segment profit (loss)									737	2,634	5,087
Depreciation and amortization	28,574	26,885	25,580	25,007	27,605	26,721	25,022	23,496	106,046	102,844	91,070
Interest expense	3,413	3,038	1,994	1,748	1,648	1,654	1,653	1,626	10,193	6,581	7,387
Interest and other income	$ (702)	$ (741)	$ (692)	$ (683)	$ (568)	$ (631)	$ (500)	$ (466)	(2,818)	(2,165)	(1,301)
Segment profit (loss)									$ 301,793	$ 275,651	$ 266,919