Business Segment Information - Reconciliation of Segment Profit to Income Before Taxes (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reconciliation of segment profit to income before income taxes
Segment profit									$ 275,651	$ 266,919	$ 259,446
Stock compensation expense	$ (9,791)	$ (12,897)	$ (13,795)	$ (13,901)	$ (14,601)	$ (11,961)	$ (9,550)	$ (4,472)	(50,384)	(40,584)	(21,252)
Changes in fair value of contingent consideration	3,017	(29,738)	(2,567)	(14,969)	(6,172)				(44,257)	(6,172)
Non-controlling interest segment profit (loss)									(2,634)	(5,087)
Depreciation and amortization	(27,605)	(26,721)	(25,022)	(23,496)	(24,405)	(23,956)	(22,480)	(20,229)	(102,844)	(91,070)	(71,994)
Interest expense	(1,648)	(1,654)	(1,653)	(1,626)	(1,668)	(2,879)	(2,004)	(836)	(6,581)	(7,387)	(3,000)
Interest and other income	568	631	500	466	474	241	275	311	2,165	1,301	1,985
Income before income taxes	$ 64,524	$ (15,015)	$ 10,280	$ 11,327	$ 35,051	$ 22,288	$ 10,588	$ 49,993	$ 71,116	$ 117,920	$ 165,185