Cash, Cash Equivalents and Marketable Securities (Details) (USD $)	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Dec. 31, 2011
Overnight Holdings [Abstract]
Money market funds (cash equivalents)	$ 135,260,000		$ 255,000,000
Investment Holdings [Abstract]
Money market funds (cash equivalents)	37,233,000		26,087,000
Marketable securities	52,686,000		16,904,000
Total cash equivalents and marketable securities	225,179,000		297,991,000
Cash held in demand accounts	18,313,000		21,732,000
Outstanding checks	(12,313,000)		(38,275,000)
Total cash, cash equivalents and marketable securities	231,179,000		281,448,000
Cash and cash equivalents	178,493,000	180,590,000	264,544,000	211,208,000
Marketable securities	52,686,000		16,904,000
Withholding associated with federal and state income taxes, employment taxes and other payroll deductions included in cash balance	117,300,000		158,200,000
Client prepayments included in cash balance	14,800,000		13,500,000
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gain on sales of available-for-sale marketable securities	0	0
Municipal bonds	52,686,000		16,904,000
Available-for-sale marketable securities [Abstract]
Amortized Cost	52,691,000		16,878,000
Gross Unrealized Gains	33,000		29,000
Gross Unrealized Losses	(38,000)		(3,000)
Municipal bonds	52,686,000		16,904,000
Contractual maturities amortized cost [Abstract]
Less than one year	28,920,000
One to five years	23,771,000
Amortized Cost	52,691,000
Contractual maturities estimated fair value [Abstract]
Less than one year	28,939,000
One to five years	23,747,000
Municipal bonds	52,686,000		16,904,000
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	172,493,000		281,087,000
Municipal bonds	0		0
Total	172,493,000		281,087,000
Available-for-sale marketable securities [Abstract]
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	52,686,000		16,904,000
Total	52,686,000		16,904,000
Available-for-sale marketable securities [Abstract]
Municipal bonds	52,686,000		16,904,000
Contractual maturities estimated fair value [Abstract]
Municipal bonds	52,686,000		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]
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	172,493,000		281,087,000
Municipal bonds	52,686,000		16,904,000
Total	225,179,000		297,991,000
Available-for-sale marketable securities [Abstract]
Municipal bonds	52,686,000		16,904,000
Contractual maturities estimated fair value [Abstract]
Municipal bonds	$ 52,686,000		$ 16,904,000