Segment Information (Details) - Reconciliation of Segment Gross Contribution to Net Income (USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total segment gross contribution	$ 248,538		$ 222,431		$ 763,108		$ 684,300
Costs and expenses:
Selling, general and administrative	181,546		164,888		547,653		492,627
Acquisition and integration charges	72		944		278		2,126
Operating income	58,743		49,768		186,735		166,046
Provision for income taxes	18,430		15,436		58,584		50,607
Net income	38,194		31,375		121,383		107,080
Reportable segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total segment gross contribution	248,538		222,431		763,108		684,300
Costs and expenses:
Selling, general and administrative	181,546		164,888		547,653		492,627
Depreciation and amortization	8,609		7,663		25,464		21,407
Acquisition and integration charges	72		944		278		2,126
Operating income	58,743		49,768		186,735		166,046
Interest expense and other	2,119		2,957		6,768		8,359
Provision for income taxes	18,430		15,436		58,584		50,607
Net income	38,194		31,375		121,383		107,080
Reportable segments | Maximum
Costs and expenses:
Percent of target bonus allocated to segments	100.00%		100.00%		100.00%		100.00%
Unallocated amount to segments
Costs and expenses:
Cost of services and product development - unallocated	$ (432)	[1]	$ (832)	[1]	$ 2,978	[1]	$ 2,094	[1]

[1] The unallocated amounts consist of certain bonus and related fringe costs recorded in Consolidated cost of services and product development expense that are not allocated to segment expense. The Company's policy is to only allocate bonus and related fringe charges to segments for up to 100% of the segment employee's target bonus. Amounts above 100% are absorbed by corporate.