Cash, Cash Equivalents and Marketable Securities (Details) (USD $)	Sep. 30, 2011	Dec. 31, 2010	Sep. 30, 2010	Dec. 31, 2009
Overnight Holdings [Abstract]
Money market funds (cash equivalents)	$ 18,367,000	$ 157,680,000
Investment Holdings [Abstract]
Money market funds (cash equivalents)	60,174,000	72,258,000
Marketable securities	55,394,000	43,367,000
Total cash equivalents and marketable securities	133,935,000	273,305,000
Cash held in demand accounts	105,194,000	31,295,000
Outstanding checks	(13,825,000)	(26,404,000)
Total cash, cash equivalents and marketable securities	225,304,000	278,196,000
Cash and cash equivalents	169,910,000	234,829,000	207,930,000	227,085,000
Withholding associated with federal and state income taxes, employment taxes and other payroll deductions included in cash balance	93,100,000	128,800,000
Client prepayments included in cash balance	3,900,000	8,100,000
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Municipal bonds	55,394,000
Schedule of Available-for-sale Securities [Line Items]
Municipal bonds	55,394,000
Contractual maturities amortized cost [Abstract]
Less than one year	31,601,000
One to five years	23,703,000
Total	55,304,000
Contractual maturities estimated fair value [Abstract]
Less than one year	31,640,000
One to five years	23,754,000
Total	55,394,000
Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	78,541,000	229,938,000
Municipal bonds	55,394,000	43,367,000
Total	133,935,000	273,305,000
Schedule of Available-for-sale Securities [Line Items]
Municipal bonds	55,394,000	43,367,000
Contractual maturities estimated fair value [Abstract]
Total	55,394,000	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	78,541,000	229,938,000
Municipal bonds	0	0
Total	78,541,000	229,938,000
Schedule of Available-for-sale Securities [Line Items]
Municipal bonds	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	55,394,000	43,367,000
Total	55,394,000	43,367,000
Schedule of Available-for-sale Securities [Line Items]
Municipal bonds	55,394,000	43,367,000
Contractual maturities estimated fair value [Abstract]
Total	55,394,000	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]
Municipal bonds	0	0
Contractual maturities estimated fair value [Abstract]
Total	0	0
US States and Political Subdivisions Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Municipal bonds	55,394,000	43,367,000
Schedule of Available-for-sale Securities [Line Items]
Municipal bonds	55,394,000	43,367,000
Amortized cost	55,304,000	43,330,000
Gross unrealized gains	125,000	63,000
Gross unrealized losses	(35,000)	(26,000)
Contractual maturities estimated fair value [Abstract]
Total	$ 55,394,000	$ 43,367,000