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, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total segment gross contribution	$ 282,833		$ 248,538		$ 880,965		$ 763,108
Costs and expenses:
Selling, general and administrative	217,025		181,546		640,179		547,653
Acquisition and integration charges	6,015		72		16,015		278
Operating income	49,391		58,743		190,322		186,735
Provision for income taxes	12,602		18,430		57,773		58,584
Net income	33,846		38,194		124,622		121,383
Reportable segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total segment gross contribution	282,833		248,538		880,965		763,108
Costs and expenses:
Selling, general and administrative	217,025		181,546		640,179		547,653
Depreciation and amortization	10,450		8,609		28,888		25,464
Acquisition and integration charges	6,015		72		16,015		278
Operating income	49,391		58,743		190,322		186,735
Interest expense and other	2,943		2,119		7,927		6,768
Provision for income taxes	12,602		18,430		57,773		58,584
Net income	33,846		38,194		124,622		121,383
Reportable segments | Maximum
Costs and expenses:
Percent of target bonus allocated to segments					100.00%
Unallocated amount to segments
Costs and expenses:
Cost of services and product development - unallocated	$ (48)	[1],[2]	$ (432)	[1],[2]	$ 5,561	[1],[2]	$ 2,978	[1],[2]

[1] The unallocated amounts consist of certain bonus and related fringe expenses 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.

[2] Amounts in parentheses in the three months ended periods result from adjustments to the year-to-date balances.