Cash, Cash Equivalents and Marketable Securities (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Overnight Holdings
Money market funds (cash equivalents)	$ 207,220,000	$ 271,840,000
Investment Holdings
Money market funds (cash equivalents)	13,882,000	14,125,000
Marketable securities	28,840,000	28,631,000
Total cash equivalents and marketable securities	249,942,000	314,596,000
Cash held in demand accounts	21,417,000	20,369,000
Outstanding checks	(13,931,000)	(29,878,000)
Total cash, cash equivalents and marketable securities	257,428,000	305,087,000
Cash and cash equivalents	228,588,000	276,456,000	213,518,000	225,755,000
Marketable securities	28,840,000	28,631,000
Withholding associated with federal and state income taxes, employment taxes and other payroll deductions included in cash balance	158,600,000	152,100,000
Client prepayments included in cash balance	22,700,000	87,900,000
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Municipal bonds	28,840,000	28,631,000
Available-for-sale marketable securities [Abstract]
Amortized Cost	28,831,000	28,626,000
Gross Unrealized Gains	15,000	16,000
Gross Unrealized Losses	(6,000)	(11,000)
Municipal bonds	28,840,000	28,631,000
Contractual maturities amortized cost [Abstract]
Less than one year	15,324,000
One to five years	13,507,000
Amortized Cost	28,831,000	28,626,000
Contractual maturities estimated fair value [Abstract]
Less than one year	15,332,000
One to five years	13,508,000
Municipal bonds	28,840,000	28,631,000
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	221,102,000	285,965,000
Municipal bonds	0	0
Total	221,102,000	285,965,000
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	28,840,000	28,631,000
Total	28,840,000	28,631,000
Available-for-sale marketable securities [Abstract]
Municipal bonds	28,840,000	28,631,000
Contractual maturities estimated fair value [Abstract]
Municipal bonds	28,840,000	28,631,000
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	221,102,000	285,965,000
Municipal bonds	28,840,000	28,631,000
Total	249,942,000	314,596,000
Available-for-sale marketable securities [Abstract]
Municipal bonds	28,840,000	28,631,000
Contractual maturities estimated fair value [Abstract]
Municipal bonds	$ 28,840,000	$ 28,631,000