Summary of Significant Accounting Policies - Summary of Impacts of Adopting ASC 606 on Condensed Balance Sheet (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Unbilled trade accounts receivable	$ 38,832	$ 33,104	$ 31,187
Other current assets	38,185		28,349
Intangible assets	72,499
Client contract costs, net of amortization	35,527
Other non-current assets	7,012		10,948
Other assets	766,102
Total assets	927,231		904,534
Deferred revenue	48,632	$ 47,611	54,231
Deferred income taxes	9,162		4,584
Other liabilities	519,577
Total liabilities	577,371		561,788
Accumulated earnings	772,071		749,438
Other stockholders' equity	(422,211)
Total stockholders' equity	349,860		342,746
Total liabilities and stockholders' equity	927,231		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	41,573
Adjustments | ASC 606
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Unbilled trade accounts receivable	(614)
Other current assets	3,670
Client contract costs, net of amortization	(35,527)
Other non-current assets	4,305
Total assets	(614)
Deferred revenue	3,218
Deferred income taxes	(179)
Total liabilities	3,039
Accumulated earnings	(3,653)
Total stockholders' equity	(3,653)
Total liabilities and stockholders' equity	(614)
Adjustments | Client contracts | ASC 606
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Intangible assets	69,125
Adjustments | Acquired client contracts | ASC 606
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Intangible assets	(41,573)
Balances without Adoption of ASC 606 | ASC 606
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Unbilled trade accounts receivable	38,218
Other current assets	41,855
Intangible assets			70,532
Other non-current assets	11,317
Other assets	766,102
Total assets	926,617
Deferred revenue	51,850
Deferred income taxes	8,983
Other liabilities	519,577
Total liabilities	580,410
Accumulated earnings	768,418
Other stockholders' equity	(422,211)
Total stockholders' equity	346,207
Total liabilities and stockholders' equity	926,617
Balances without Adoption of ASC 606 | Client contracts | ASC 606
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Intangible assets	$ 69,125		43,626
Balances without Adoption of ASC 606 | Acquired client contracts | ASC 606
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Intangible assets			$ 10,020