Summary of Significant Accounting Policies - Summary of Impacts of Adopting ASC 606 on Condensed Balance Sheet (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Unbilled trade accounts receivable	$ 35,426	$ 33,104	$ 31,187
Other current assets	32,388		28,349
Intangible assets	70,582
Client contract costs, net of amortization	38,357
Other non-current assets	7,963		10,948
Other assets	774,616
Total assets	928,438		904,534
Deferred revenue	47,388	$ 47,611	54,231
Deferred income taxes	8,412		4,584
Other liabilities	514,563
Total liabilities	570,363		561,788
Accumulated earnings	764,015		749,438
Other stockholders' equity	(405,940)
Total stockholders' equity	358,075		342,746
Total liabilities and stockholders' equity	928,438		904,534
Client contracts
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Intangible assets			43,626
Acquired client contracts
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Intangible assets	39,688
Adjustments | ASU 606
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Unbilled trade accounts receivable	(697)
Other current assets	3,184
Client contract costs, net of amortization	(38,357)
Other non-current assets	4,485
Total assets	(697)
Deferred revenue	3,781
Deferred income taxes	(366)
Total liabilities	3,415
Accumulated earnings	(4,112)
Total stockholders' equity	(4,112)
Total liabilities and stockholders' equity	(697)
Adjustments | Client contracts | ASU 606
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Intangible assets	70,376
Adjustments | Acquired client contracts | ASU 606
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Intangible assets	(39,688)
Balances without Adoption of ASC 606 | ASU 606
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Unbilled trade accounts receivable	34,729
Other current assets	35,572
Intangible assets			70,532
Other non-current assets	12,448
Other assets	774,616
Total assets	927,741
Deferred revenue	51,169
Deferred income taxes	8,046
Other liabilities	514,563
Total liabilities	573,778
Accumulated earnings	759,903
Other stockholders' equity	(405,940)
Total stockholders' equity	353,963
Total liabilities and stockholders' equity	927,741
Balances without Adoption of ASC 606 | Client contracts | ASU 606
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Intangible assets	$ 70,376		43,626
Balances without Adoption of ASC 606 | Acquired client contracts | ASU 606
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Intangible assets			$ 10,020