Consolidated Statements of Cash Flows - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities
Net income		$ 740,200	$ 452,385	$ 362,431
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation		46,599	36,456	30,462
Stock-based compensation		93,297	63,946	90,122
Provision for losses on accounts receivable		44,857	35,885	24,629
Amortization of deferred financing costs and discounts		6,952	7,582	7,049
Amortization of intangible assets		211,849	161,635	159,740
Amortization of premium on receivables		6,112	5,165	3,250
Loss on extinguishment of debt		3,296	0	0
Deferred income taxes		(247,712)	(28,681)	30,626
Investment loss		53,164	36,356	57,668
Gain on disposition of business		(174,983)	0	0
Other non-cash operating income		(61)	(690)	(4,242)
Changes in operating assets and liabilities (net of acquisitions and disposition):
Restricted cash		(4,335)	(2,306)	(35,676)
Accounts receivable and other receivables		(431,003)	(338,796)	40,017
Prepaid expenses and other current assets		26,102	5,301	(12,564)
Other assets		(20,957)	(20,345)	(2,524)
Excess tax benefits related to stock-based compensation		0	0	(26,427)
Accounts payable, accrued expenses and customer deposits		322,346	292,019	30,023
Net cash provided by operating activities		675,723	705,912	754,584
Investing activities
Acquisitions, net of cash acquired1	[1]	(705,257)	(1,331,985)	(49,069)
Purchases of property and equipment		(70,093)	(59,011)	(41,875)
Proceeds from disposal of a business		316,501	0	0
Other		(38,953)	1,411	(8,470)
Net cash used in investing activities		(497,802)	(1,389,585)	(99,414)
Financing activities
Excess tax benefits related to stock-based compensation		0	0	26,427
Proceeds from issuance of common stock		44,690	21,231	19,926
Borrowings (payments) on securitization facility, net		220,000	(23,000)	(61,000)
Repurchase of common stock		(402,393)	(187,678)	0
Deferred financing costs paid and debt discount		(12,908)	(2,272)	0
Proceeds from issuance of notes payable		780,656	600,000	0
Principal payments on notes payable		(423,156)	(118,500)	(103,500)
Borrowings from revolver		1,100,000	1,225,107	0
Payments on revolver		(1,031,722)	(786,849)	(486,818)
Borrowings (payments) on swing line of credit, net		(23,686)	26,606	(546)
Payment of contingent consideration		0	0	(42,177)
Other		457	(676)	(377)
Net cash provided by (used in) financing activities		251,938	753,969	(648,065)
Effect of foreign currency exchange rates on cash		8,718	(42,430)	(37,022)
Net increase (decrease) in cash		438,577	27,866	(29,917)
Cash and cash equivalents, beginning of year		475,018	447,152	477,069
Cash and cash equivalents, end of year		913,595	475,018	447,152
Supplemental cash flow information
Cash paid for interest		113,416	70,339	72,537
Cash paid for income taxes		392,192	101,951	83,380
Non cash investing activity, notes assumed in acquisitions		$ 29,341	$ 0	$ 0

[1] Amounts reported in acquisitions and investment, net of cash acquired, includes debt assumed and immediately repaid in acquisitions.