Summary of Significant Accounting Policies - Summary of Impacts of Adopting ASC 606 on Condensed Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Unbilled trade accounts receivable	$ 37,227	$ 33,104	$ 31,187
Other current assets	32,286		28,349
Intangible assets	101,856
Client contract costs, net of amortization	37,289
Other non-current assets	26,934		10,948
Other assets	915,170
Total assets	1,114,362		904,534
Deferred revenue	57,763	$ 47,611	54,231
Deferred income taxes	8,205		4,584
Other liabilities	687,370
Total liabilities	753,338		561,788
Accumulated earnings	795,127		749,438
Other stockholders' equity	(434,103)
Total stockholders' equity	361,024		342,746	$ 251,360	$ 345,845
Total liabilities and stockholders' equity	1,114,362		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	65,456
Adjustments | ASC 606
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Unbilled trade accounts receivable	(451)
Other current assets	5,834
Client contract costs, net of amortization	(37,289)
Other non-current assets	8,178
Total assets	(451)
Deferred revenue	2,463
Deferred income taxes	87
Total liabilities	2,550
Accumulated earnings	(3,001)
Total stockholders' equity	(3,001)
Total liabilities and stockholders' equity	(451)
Adjustments | Client contracts | ASC 606
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Intangible assets	88,733
Adjustments | Acquired client contracts | ASC 606
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Intangible assets	(65,456)
Balances without Adoption of ASC 606 | ASC 606
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Unbilled trade accounts receivable	36,776
Other current assets	38,120
Intangible assets			70,532
Other non-current assets	35,112
Other assets	915,170
Total assets	1,113,911
Deferred revenue	60,226
Deferred income taxes	8,292
Other liabilities	687,370
Total liabilities	755,888
Accumulated earnings	792,126
Other stockholders' equity	(434,103)
Total stockholders' equity	358,023
Total liabilities and stockholders' equity	1,113,911
Balances without Adoption of ASC 606 | Client contracts | ASC 606
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Intangible assets	$ 88,733		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