Cash, Cash Equivalents and Marketable Securities (Details) (USD $)	6 Months Ended	12 Months Ended
	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013	Dec. 31, 2012
Overnight Holdings
Money market funds (cash equivalents)	$ 114,820,000	$ 192,040,000
Investment Holdings
Money market funds (cash equivalents)	45,784,000	42,913,000
Marketable securities	43,431,000	46,340,000
Total cash equivalents and marketable securities	204,035,000	281,293,000
Cash held in demand accounts	18,250,000	23,054,000
Outstanding checks	(9,965,000)	(32,252,000)
Total cash, cash equivalents and marketable securities	212,320,000	272,095,000
Cash and cash equivalents	168,889,000	225,755,000	178,493,000	264,544,000
Marketable securities	43,431,000	46,340,000
Withholding associated with federal and state income taxes, employment taxes and other payroll deductions included in cash balance	95,300,000	143,000,000
Client prepayments included in cash balance	13,700,000	24,500,000
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Municipal bonds	43,431,000	46,340,000
Available-for-sale marketable securities [Abstract]
Amortized Cost	43,362,000	46,290,000
Gross Unrealized Gains	71,000	51,000
Gross Unrealized Losses	(2,000)	(1,000)
Municipal bonds	43,431,000	46,340,000
Contractual maturities amortized cost [Abstract]
Less than one year	20,400,000
One to five years	22,962,000
Amortized Cost	43,362,000
Contractual maturities estimated fair value [Abstract]
Less than one year	20,413,000
One to five years	23,018,000
Municipal bonds	43,431,000	46,340,000
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	160,604,000	234,953,000
Municipal bonds	0	0
Total	160,604,000	234,953,000
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	43,431,000	46,340,000
Total	43,431,000	46,340,000
Available-for-sale marketable securities [Abstract]
Municipal bonds	43,431,000	46,340,000
Contractual maturities estimated fair value [Abstract]
Municipal bonds	43,431,000	46,340,000
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	160,604,000	234,953,000
Municipal bonds	43,431,000	46,340,000
Total	204,035,000	281,293,000
Available-for-sale marketable securities [Abstract]
Municipal bonds	43,431,000	46,340,000
Contractual maturities estimated fair value [Abstract]
Municipal bonds	$ 43,431,000	$ 46,340,000