Cash, Cash Equivalents and Marketable Securities (Details) (USD $)	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011	Dec. 31, 2010
Overnight Holdings [Abstract]
Money market funds (cash equivalents)	$ 26,579,000	$ 71,350,000
Investment Holdings [Abstract]
Money market funds (cash equivalents)	59,348,000	59,587,000
Marketable securities	57,182,000	56,987,000
Total cash equivalents and marketable securities	143,109,000	187,924,000
Cash held in demand accounts	110,920,000	113,968,000
Outstanding checks	(16,257,000)	(33,697,000)
Total cash, cash equivalents and marketable securities	237,772,000	268,195,000
Cash and cash equivalents	180,590,000	211,208,000	265,191,000	234,829,000
Marketable securities	57,182,000	56,987,000
Total cash, cash equivalents and marketable securities	237,772,000	268,195,000
Withholding associated with federal and state income taxes, employment taxes and other payroll deductions included in cash balance	111,900,000	150,800,000
Client prepayments included in cash balance	10,500,000	10,400,000
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Municipal bonds	57,182,000
Schedule of Available-for-sale Securities [Line Items]
Municipal bonds	57,182,000
Contractual maturities amortized cost [Abstract]
Less than one year	30,597,000
One to five years	26,486,000
Total	57,083,000
Contractual maturities estimated fair value [Abstract]
Less than one year	30,620,000
One to five years	26,562,000
Total	57,182,000
US States and Political Subdivisions Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Municipal bonds	57,182,000	56,987,000
Schedule of Available-for-sale Securities [Line Items]
Municipal bonds	57,182,000	56,987,000
Amortized cost	57,083,000	56,945,000
Gross unrealized gains	114,000	90,000
Gross unrealized losses	(15,000)	(48,000)
Contractual maturities estimated fair value [Abstract]
Total	57,182,000	56,987,000
Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	85,927,000	130,937,000
Municipal bonds	57,182,000	56,987,000
Total	143,109,000	187,924,000
Schedule of Available-for-sale Securities [Line Items]
Municipal bonds	57,182,000	56,987,000
Contractual maturities estimated fair value [Abstract]
Total	57,182,000	56,987,000
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	85,927,000	130,937,000
Municipal bonds	0	0
Total	85,927,000	130,937,000
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	57,182,000	56,987,000
Total	57,182,000	56,987,000
Schedule of Available-for-sale Securities [Line Items]
Municipal bonds	57,182,000	56,987,000
Contractual maturities estimated fair value [Abstract]
Total	57,182,000	56,987,000
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