Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash flows from operating activities:
Net income	$ 335,691	$ 317,203	$ 280,517
Adjustments to reconcile net income to net cash provided from operating activities:
Depreciation and amortization	64,228	45,990	39,966
Net change in fair value of contingent consideration liabilities	13	(417)	3,656
Stock-based compensation expense	32,129	31,719	31,288
Equity in earnings of unconsolidated affiliate	(3,127)	(1,780)
Distribution of earnings from unconsolidated affiliate	2,062
Deferred income taxes	14,494	2,559	15,920
Changes in assets and liabilities:
Accounts receivable	(55,391)	(57,018)	(37,331)
Prepaid expenses and other assets	(4,905)	1,506	212
Other assets	1,639	2,137	(3,541)
Accounts payable and accrued expenses	(7,874)	66,039	49,239
Income taxes payable	(4,101)	6,998	5,772
Payments of contingent consideration liabilities	(1,439)	(4,071)	(1,308)
Long-term professional liabilities	(2,064)	9,284	19,797
Other liabilities	(2,654)	2,492	1,211
Net cash provided from operating activities	368,701	422,641	405,398
Cash flows from investing activities:
Acquisition payments, net of cash acquired	(818,903)	(479,394)	(238,045)
Purchases of investments	(33,980)	(26,884)	(20,802)
Proceeds from maturities of investments	31,956	20,735	11,011
Purchases of property and equipment	(27,073)	(18,061)	(15,654)
Net cash used in investing activities	(848,000)	(503,604)	(263,490)
Cash flows from financing activities:
Borrowings on credit facility	2,121,500	1,754,500	911,500
Payments on credit facility	(2,156,000)	(1,213,500)	(1,028,500)
Proceeds from issuance of senior notes	750,000
Payments for financing costs	(14,190)	(4,281)
Payments of contingent consideration liabilities	(12,856)	(11,740)	(10,365)
Payments on capital lease obligations	(2,171)	(159)	(99)
Excess tax benefit from exercises of stock options and vesting of restricted and deferred stock	11,583	17,462	18,579
Proceeds from issuance of common stock	20,128	42,876	28,697
Contribution from noncontrolling interests		1,025
Repurchases of common stock	(235,051)	(488,429)	(51,863)
Net cash provided from (used in) financing activities	482,943	97,754	(132,051)
Net increase in cash and cash equivalents	3,644	16,791	9,857
Cash and cash equivalents at beginning of year	47,928	31,137	21,280
Cash and cash equivalents at end of year	51,572	47,928	31,137
Supplemental disclosure of cash flow information:
Interest	20,367	7,323	3,312
Income taxes	181,005	161,841	$ 126,411
Non-cash investing and financing activities:
Value of common stock issued for an acquisition	7,800
Equipment financed through capital leases	3,135	$ 1,244
Property and equipment included in accounts payable	$ 1,800