Segment Information (Details) - Reconciliation of Segment Gross Contribution to Net Income (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total segment gross contribution	$ 269,710		$ 237,771		$ 514,569		$ 461,869
Costs and expenses:
Selling, general and administrative	185,629		165,221		366,107		327,739
Acquisition and integration charges	106		1,182		206		1,182
Operating income	73,987		62,722		127,992		116,278
Provision for income taxes	25,049		19,009		40,154		35,171
Net income	46,514		41,484		83,189		75,705
Reportable segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total segment gross contribution	269,710		237,771		514,569		461,869
Costs and expenses:
Selling, general and administrative	185,629		165,221		366,107		327,739
Depreciation and amortization	8,421		7,110		16,855		13,744
Acquisition and integration charges	106		1,182		206		1,182
Operating income	73,987		62,722		127,992		116,278
Interest expense and other	2,424		2,229		4,649		5,402
Provision for income taxes	25,049		19,009		40,154		35,171
Net income	46,514		41,484		83,189		75,705
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	$ 1,567	[1]	$ 1,536	[1]	$ 3,409	[1]	$ 2,926	[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.