Summary of Significant Accounting Policies - Summary of Impacts of Adopting ASC 606 on Condensed Statement of Cash Flows (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Net income	$ 45,232	$ 46,398
Adjustments to reconcile net income to net cash provided by operating activities-
Amortization	31,974	21,670
Deferred income taxes	2,150	1,487
Other	32,255
Changes in operating assets and liabilities:
Other current and non-current assets	(21,763)	(1,788)
Deferred revenue	7,182	10,940
Other	(23,761)
Net cash provided by operating activities	73,269	102,822
Cash flows from investing activities:
Acquisition of and investments in client contracts		(10,082)
Other	21,692
Net cash provided by investing activities	21,692	1,113
Net cash used in financing activities	(18,770)	$ (89,931)
Effect of exchange rate fluctuations on cash	(1,262)
Net increase cash and cash equivalents	74,929
Cash and cash equivalents, beginning of period	122,243
Cash and cash equivalents, end of period	197,172
Adjustments | ASC 606
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Net income	1,373
Adjustments to reconcile net income to net cash provided by operating activities-
Amortization	(3,146)
Deferred income taxes	561
Changes in operating assets and liabilities:
Other current and non-current assets	10,231
Deferred revenue	(1,689)
Net cash provided by operating activities	7,330
Cash flows from investing activities:
Acquisition of and investments in client contracts	(7,330)
Net cash provided by investing activities	(7,330)
Balances without Adoption of ASC 606 | ASC 606
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Net income	46,605
Adjustments to reconcile net income to net cash provided by operating activities-
Amortization	28,828
Deferred income taxes	2,711
Other	32,255
Changes in operating assets and liabilities:
Other current and non-current assets	(11,532)
Deferred revenue	5,493
Other	(23,761)
Net cash provided by operating activities	80,599
Cash flows from investing activities:
Acquisition of and investments in client contracts	(7,330)
Other	21,692
Net cash provided by investing activities	14,362
Net cash used in financing activities	(18,770)
Effect of exchange rate fluctuations on cash	(1,262)
Net increase cash and cash equivalents	74,929
Cash and cash equivalents, beginning of period	122,243
Cash and cash equivalents, end of period	$ 197,172