Summary of Significant Accounting Policies - Summary of Impacts of Adopting ASC 606 on Condensed Balance Sheet (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Unbilled trade accounts receivable	$ 36,847	$ 33,104	$ 31,187
Other current assets	38,706		28,349
Intangible assets	71,816
Client contract costs, net of amortization	35,584
Other non-current assets	10,898		10,948
Other assets	785,411
Total assets	947,309		904,534
Deferred revenue	54,966	$ 47,611	54,231
Deferred income taxes	5,881		4,584
Other liabilities	528,043
Total liabilities	588,890		561,788
Accumulated earnings	781,188		749,438
Other stockholders' equity	(422,769)
Total stockholders' equity	358,419		342,746
Total liabilities and stockholders' equity	947,309		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	39,863
Adjustments | ASC 606
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Unbilled trade accounts receivable	(533)
Other current assets	4,014
Client contract costs, net of amortization	(35,584)
Other non-current assets	5,069
Total assets	(533)
Deferred revenue	2,655
Deferred income taxes	8
Total liabilities	2,663
Accumulated earnings	(3,196)
Total stockholders' equity	(3,196)
Total liabilities and stockholders' equity	(533)
Adjustments | Client contracts | ASC 606
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Intangible assets	66,364
Adjustments | Acquired client contracts | ASC 606
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Intangible assets	(39,863)
Balances without Adoption of ASC 606 | ASC 606
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Unbilled trade accounts receivable	36,314
Other current assets	42,720
Intangible assets			70,532
Other non-current assets	15,967
Other assets	785,411
Total assets	946,776
Deferred revenue	57,621
Deferred income taxes	5,889
Other liabilities	528,043
Total liabilities	591,553
Accumulated earnings	777,992
Other stockholders' equity	(422,769)
Total stockholders' equity	355,223
Total liabilities and stockholders' equity	946,776
Balances without Adoption of ASC 606 | Client contracts | ASC 606
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Intangible assets	$ 66,364		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