Cash, Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014	Dec. 31, 2013
Overnight Holdings
Money market funds (cash equivalents)	$ 130,780	$ 271,840
Investment Holdings
Money market funds (cash equivalents)	29,174	14,125
Marketable securities	21,648	28,631
Total cash equivalents and marketable securities	181,602	314,596
Cash held in demand accounts	17,852	20,369
Outstanding checks	(10,078)	(29,878)
Total cash, cash equivalents and marketable securities	189,376	305,087
Cash and cash equivalents	167,728	276,456	$ 168,889	$ 225,755
Marketable securities	21,648	28,631
Withholding associated with federal and state income taxes, employment taxes and other payroll deductions included in cash balance	106,200	152,100
Client prepayments included in cash balance	19,400	87,900
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	159,954	285,965
Municipal bonds	21,648	28,631
Total	181,602	314,596
Available-for-sale marketable securities [Abstract]
Amortized Cost	21,649	28,626
Gross Unrealized Gains	6	16
Gross Unrealized Losses	(7)	(11)
Municipal bonds	21,648	28,631
Contractual maturities amortized cost [Abstract]
Less than one year	13,992
One to five years	7,657
Amortized Cost	21,649	28,626
Contractual maturities estimated fair value [Abstract]
Less than one year	13,996
One to five years	7,652
Municipal bonds	21,648	28,631
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	159,954	285,965
Municipal bonds	0	0
Total	159,954	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	21,648	28,631
Total	21,648	28,631
Available-for-sale marketable securities [Abstract]
Municipal bonds	21,648	28,631
Contractual maturities estimated fair value [Abstract]
Municipal bonds	21,648	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