Cash, Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
Overnight Holdings
Money market funds (cash equivalents)	$ 173,586	$ 271,840
Investment Holdings
Money market funds (cash equivalents)	21,301	14,125
Marketable securities	9,754	28,631
Total cash equivalents and marketable securities	204,641	314,596
Cash held in demand accounts	17,748	20,369
Outstanding checks	(11,550)	(29,878)
Total cash, cash equivalents and marketable securities	210,839	305,087
Cash and cash equivalents	201,085	276,456	$ 179,458	$ 225,755
Marketable securities	9,754	28,631
Withholding associated with federal and state income taxes, employment taxes and other payroll deductions included in cash balance	99,400	152,100
Client prepayments included in cash balance	40,500	87,900
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	194,887	285,965
Municipal bonds	9,754	28,631
Total	204,641	314,596
Available-for-sale marketable securities [Abstract]
Amortized Cost	9,746	28,626
Gross Unrealized Gains	10	16
Gross Unrealized Losses	(2)	(11)
Municipal bonds	9,754	28,631
Contractual maturities amortized cost [Abstract]
Less than one year	6,294
One to five years	3,452
Amortized Cost	9,746	28,626
Contractual maturities estimated fair value [Abstract]
Less than one year	6,297
One to five years	3,457
Municipal bonds	9,754	28,631
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	194,887	285,965
Municipal bonds	0	0
Total	194,887	285,965
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	9,754	28,631
Total	9,754	28,631
Available-for-sale marketable securities [Abstract]
Municipal bonds	9,754	28,631
Contractual maturities estimated fair value [Abstract]
Municipal bonds	9,754	28,631
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