Cash, Cash Equivalents and Marketable Securities (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Overnight Holdings [Abstract]
Money market funds (cash equivalents)	$ 255,000,000	$ 71,350,000
Investment Holdings [Abstract]
Money market funds (cash equivalents)	26,087,000	59,587,000
Marketable securities	16,904,000	56,987,000
Total cash equivalents and marketable securities	297,991,000	187,924,000
Cash held in demand accounts	21,732,000	113,968,000
Outstanding checks	(38,275,000)	(33,697,000)
Total cash, cash equivalents and marketable securities	281,448,000	268,195,000
Cash and cash equivalents	264,544,000	211,208,000	234,829,000	227,085,000
Marketable securities	16,904,000	56,987,000
Withholding associated with federal and state income taxes, employment taxes and other payroll deductions included in cash balance	158,200,000	150,800,000
Client prepayments included in cash balance	13,500,000	10,400,000
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	281,087,000	130,937,000
Municipal bonds	16,904,000	56,987,000
Total	297,991,000	187,924,000
Schedule of Available-for-sale Securities [Line Items]
Municipal bonds	16,904,000	56,987,000
Gain on sales of available-for-sale marketable securities	58,000,000
Contractual maturities amortized cost [Abstract]
Less than one year	9,301,000
One to five years	7,577,000
Amortized cost	16,878,000
Contractual maturities estimated fair value [Abstract]
Less than one year	9,309,000
One to five years	7,595,000
Municipal bonds	16,904,000	56,987,000
U.S. Government securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Municipal bonds	16,904,000	56,987,000
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	16,878,000	56,945,000
Gross unrealized gains	29,000	90,000
Gross unrealized losses	(3,000)	(48,000)
Municipal bonds	16,904,000	56,987,000
Contractual maturities estimated fair value [Abstract]
Municipal bonds	16,904,000	56,987,000
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	281,087,000	130,937,000
Municipal bonds	0	0
Total	281,087,000	130,937,000
Schedule of Available-for-sale Securities [Line Items]
Municipal bonds	0	0
Contractual maturities estimated fair value [Abstract]
Municipal bonds	0	0
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	0	0
Municipal bonds	16,904,000	56,987,000
Total	16,904,000	56,987,000
Schedule of Available-for-sale Securities [Line Items]
Municipal bonds	16,904,000	56,987,000
Contractual maturities estimated fair value [Abstract]
Municipal bonds	16,904,000	56,987,000
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]
Municipal bonds	0	0
Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	281,087,000	130,937,000
Municipal bonds	16,904,000	56,937,000
Total	297,991,000	181,924,000
Schedule of Available-for-sale Securities [Line Items]
Municipal bonds	16,904,000	56,937,000
Contractual maturities estimated fair value [Abstract]
Municipal bonds	$ 16,904,000	$ 56,937,000