Cash, Cash Equivalents and Marketable Securities (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Dec. 31, 2011
Overnight Holdings [Abstract]
Money market funds (cash equivalents)	$ 178,040,000		$ 255,000,000
Investment Holdings [Abstract]
Money market funds (cash equivalents)	38,722,000		26,087,000
Marketable securities	51,397,000		16,904,000
Total cash equivalents and marketable securities	268,159,000		297,991,000
Cash held in demand accounts	22,345,000		21,732,000
Outstanding checks	(11,675,000)		(38,275,000)
Total cash, cash equivalents and marketable securities	278,829,000		281,448,000
Cash and cash equivalents	227,432,000	234,548,000	264,544,000	211,208,000
Marketable securities	51,397,000		16,904,000
Withholding associated with federal and state income taxes, employment taxes and other payroll deductions included in cash balance	169,600,000		158,200,000
Client prepayments included in cash balance	11,900,000		13,500,000
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Municipal bonds	51,397,000		16,904,000
Available-for-sale marketable securities [Abstract]
Amortized Cost	51,342,000		16,878,000
Gross Unrealized Gains	56,000		29,000
Gross Unrealized Losses	(1,000)		(3,000)
Estimated Fair Value	51,397,000		16,904,000
Gain on sales of available-for-sale marketable securities	0	0
Contractual maturities amortized cost [Abstract]
Less than one year	23,231,000
One to five years	28,111,000
Amortized Cost	51,342,000
Contractual maturities estimated fair value [Abstract]
Less than one year	23,255,000
One to five years	28,142,000
Estimated Fair Value	51,397,000		16,904,000
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	216,762,000		281,087,000
Municipal bonds	0		0
Total	216,762,000		281,087,000
Available-for-sale marketable securities [Abstract]
Estimated Fair Value	0		0
Contractual maturities estimated fair value [Abstract]
Estimated Fair Value	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	51,397,000		16,904,000
Total	51,397,000		16,904,000
Available-for-sale marketable securities [Abstract]
Estimated Fair Value	51,397,000		16,904,000
Contractual maturities estimated fair value [Abstract]
Estimated Fair Value	51,397,000		16,904,000
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
Available-for-sale marketable securities [Abstract]
Estimated Fair Value	0		0
Contractual maturities estimated fair value [Abstract]
Estimated Fair Value	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	216,762,000		281,087,000
Municipal bonds	51,397,000		16,904,000
Total	268,159,000		297,991,000
Available-for-sale marketable securities [Abstract]
Estimated Fair Value	51,397,000		16,904,000
Contractual maturities estimated fair value [Abstract]
Estimated Fair Value	$ 51,397,000		$ 16,904,000