Summary of Significant Accounting Policies (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Summary of Significant Accounting Policies [Abstract]
Number of states in which customers are located	50
Property, Plant and Equipment [Line Items]
Property and equipment, net	$ 156,494,000	$ 142,188,000
Depreciation	39,100,000	36,600,000	40,400,000
Finite-Lived Intangible Assets [Line Items]
Intangible assets not subject to amortization	29,000,000	29,000,000
Revenue Recognition [Abstract]
Percentage of performance-based revenues (in hundredths)	7.00%
Cumulative performance-based revenues not settled yet but recognized	41,300,000
Net increase in revenue relating to services provided in prior periods	9,200,000	2,900,000	25,800,000
Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Estimated useful life	1 year
Maximum [Member]
Finite-Lived Intangible Assets [Line Items]
Estimated useful life	25 years
Computer Software and Hardware [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Useful life	3 years
Computer Software and Hardware [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Useful life	7 years
Furniture and Other Office Equipment [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Useful life	4 years
Furniture and Other Office Equipment [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Useful life	7 years
Leasehold Improvements [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Useful life	2 years
Leasehold Improvements [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Useful life	15 years
Computer Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, net	103,700,000	76,700,000
Depreciation	$ 24,900,000	$ 21,400,000	$ 24,300,000