Cash, Cash Equivalents and Marketable Securities (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2011
Overnight Holdings
Money market funds (cash equivalents)	$ 116,580,000	$ 255,000,000
Investment Holdings
Money market funds (cash equivalents)	37,877,000	26,087,000
Marketable securities	47,400,000	16,904,000
Total cash equivalents and marketable securities	201,857,000	297,991,000
Cash held in demand accounts	18,717,000	21,732,000
Outstanding checks	(11,332,000)	(38,275,000)
Total cash, cash equivalents and marketable securities	209,242,000	281,448,000
Cash and cash equivalents	161,842,000	264,544,000	202,218,000	211,208,000
Marketable securities	47,400,000	16,904,000
Withholding associated with federal and state income taxes, employment taxes and other payroll deductions included in cash balance	90,100,000	158,200,000
Client prepayments included in cash balance	9,000,000	13,500,000
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Municipal bonds	47,400,000	16,904,000
Available-for-sale marketable securities [Abstract]
Amortized Cost	47,361,000	16,878,000
Gross Unrealized Gains	41,000	29,000
Gross Unrealized Losses	(2,000)	(3,000)
Municipal bonds	47,400,000	16,904,000
Contractual maturities amortized cost [Abstract]
Less than one year	30,885,000
One to five years	16,476,000
Amortized Cost	47,361,000
Contractual maturities estimated fair value [Abstract]
Less than one year	30,905,000
One to five years	16,495,000
Municipal bonds	47,400,000	16,904,000
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	154,457,000	281,087,000
Municipal bonds	0	0
Total	154,457,000	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	47,400,000	16,904,000
Total	47,400,000	16,904,000
Available-for-sale marketable securities [Abstract]
Municipal bonds	47,400,000	16,904,000
Contractual maturities estimated fair value [Abstract]
Municipal bonds	47,400,000	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	154,457,000	281,087,000
Municipal bonds	47,400,000	16,904,000
Total	201,857,000	297,991,000
Available-for-sale marketable securities [Abstract]
Municipal bonds	47,400,000	16,904,000
Contractual maturities estimated fair value [Abstract]
Municipal bonds	$ 47,400,000	$ 16,904,000