Cash, Cash Equivalents and Marketable Securities (Details) (USD $)	Jun. 30, 2011	Dec. 31, 2010	Jun. 30, 2010	Dec. 31, 2009
Overnight Holdings
Money market funds (cash equivalents)	$ 82,413,000	$ 157,680,000
Investment Holdings [Abstract]
Money market funds (cash equivalents)	87,952,000	72,258,000
Marketable securities	37,598,000	43,367,000
Total cash equivalents and marketable securities	207,963,000	273,305,000
Cash held in demand accounts	105,672,000	31,295,000
Outstanding checks	(10,846,000)	(26,404,000)
Total cash, cash equivalents and marketable securities	302,789,000	278,196,000
Cash and cash equivalents	265,191,000	234,829,000	169,382,000	227,085,000
Withholding associated with federal and state income taxes, employment taxes and other payroll deductions included in cash balance. (in millions)	83,600,000	128,800,000
Client prepayments included in cash balance (in millions)	81,100,000	8,100,000
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Municipal bonds	37,598,000
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value	37,598,000
Less than one year	20,369,000
One to five years	17,128,000
Total	37,497,000
Less than one year	20,395,000
One to five years	17,203,000
Total	37,598,000
Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	170,365,000	229,938,000
Municipal bonds	37,598,000	43,367,000
Total	207,963,000	273,305,000
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value	37,598,000	43,367,000
Total	37,598,000	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		229,938,000	170,365,000
Municipal bonds		0	0
Total		229,938,000	170,365,000
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value		0	0
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	37,598,000		43,367,000
Total	37,598,000		43,367,000
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value	37,598,000		43,367,000
Total	37,598,000		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
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	37,598,000	43,367,000
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	37,497,000	43,330,000
Gross unrealied gains	110,000	63,000
Gross unrealized losses	(9,000)	(26,000)
Estimated fair value	37,598,000	43,367,000
Total	$ 37,598,000	$ 43,367,000