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, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total segment gross contribution	$ 318,730		$ 269,710		$ 598,133		$ 514,569
Costs and expenses:
Selling, general and administrative	218,537		185,629		423,154		366,107
Acquisition and integration charges	6,644		106		10,000		206
Operating income	81,761		73,987		140,931		127,992
Provision for income taxes	26,216		25,049		45,171		40,154
Net income	53,040		46,514		90,776		83,189
Reportable segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total segment gross contribution	318,730		269,710		598,133		514,569
Costs and expenses:
Selling, general and administrative	218,537		185,629		423,154		366,107
Depreciation and amortization	9,700		8,421		18,438		16,855
Acquisition and integration charges	6,644		106		10,000		206
Operating income	81,761		73,987		140,931		127,992
Interest expense and other	2,505		2,424		4,984		4,649
Provision for income taxes	26,216		25,049		45,171		40,154
Net income	53,040		46,514		90,776		83,189
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	$ 2,088	[1]	$ 1,567	[1]	$ 5,610	[1]	$ 3,409	[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.