Cash, Cash Equivalents and Marketable Securities (Details) (USD $)	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Overnight Holdings [Abstract]
Money market funds (cash equivalents)	$ 71,350,000	$ 157,680,000
Investment Holdings [Abstract]
Money market funds (cash equivalents)	59,587,000	72,258,000
Marketable securities	56,987,000	43,367,000
Total cash equivalents and marketable securities	187,924,000	273,305,000
Cash held in demand accounts	113,968,000	31,295,000
Outstanding checks	(33,697,000)	(26,404,000)
Total cash, cash equivalents and marketable securities	268,195,000	278,196,000
Withholding associated with federal and state income taxes, employment taxes and other payroll deductions included in cash balance	150,800,000	128,800,000
Client prepayments included in cash balance	10,400,000	8,100,000
Cash and cash equivalents	211,208,000	234,829,000	227,085,000	252,190,000
Marketable securities	56,987,000	43,367,000
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Municipal bonds	56,987,000
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value	56,987,000
Contractual maturities amortized cost [Abstract]
Less than one year	33,722,000
One to five years	23,223,000
Total	56,945,000
Contractual maturities estimated fair value [Abstract]
Less than one year	33,701,000
One to five years	23,286,000
Total	56,987,000
US States and Political Subdivisions Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Municipal bonds	56,987,000	43,367,000
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	56,945,000	43,330,000
Gross unrealized gains	90,000	63,000
Gross unrealized losses	(48,000)	(26,000)
Estimated fair value	56,987,000	43,367,000
Contractual maturities estimated fair value [Abstract]
Total	56,987,000	43,367,000
Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	130,937,000	229,938,000
Municipal bonds	56,987,000	43,367,000
Total	187,924,000	273,305,000
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value	56,987,000	43,367,000
Contractual maturities estimated fair value [Abstract]
Total	56,987,000	43,367,000
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	130,937,000	229,938,000
Municipal bonds	0	0
Total	130,937,000	229,938,000
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value	0	0
Contractual maturities estimated fair value [Abstract]
Total	0	0
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	0	0
Municipal bonds	56,987,000	43,367,000
Total	56,987,000	43,367,000
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value	56,987,000	43,367,000
Contractual maturities estimated fair value [Abstract]
Total	56,987,000	43,367,000
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	0	0
Municipal bonds	0	0
Total	0	0
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value	0	0
Contractual maturities estimated fair value [Abstract]
Total	$ 0	$ 0