Cash, Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015	Dec. 31, 2014
Overnight Holdings
Money market funds (cash equivalents)	$ 233,080	$ 247,720
Investment Holdings
Money market funds (cash equivalents)	24,010	26,048
Marketable securities	1,881	9,875
Total cash equivalents and marketable securities	258,971	283,643
Cash held in demand accounts	24,666	19,377
Outstanding checks	(10,183)	(23,607)
Total cash, cash equivalents and marketable securities	273,454	279,413
Cash and cash equivalents	271,573	269,538	$ 167,728	$ 276,456
Marketable securities	1,881	9,875
Withholding associated with federal and state income taxes, employment taxes and other payroll deductions included in cash balance	121,400	185,700
Client prepayments included in cash balance	100,700	17,000
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	257,090	273,768
Municipal bonds	1,881	9,875
Total	258,971	283,643
Available-for-sale marketable securities [Abstract]
Amortized Cost	1,879	9,875
Gross Unrealized Gains	2	3
Gross Unrealized Losses	0	(3)
Municipal bonds	1,881	9,875
Contractual maturities amortized cost [Abstract]
Less than one year	1,351
One to five years	528
Amortized Cost	1,879	9,875
Contractual maturities estimated fair value [Abstract]
Less than one year	1,351
One to five years	530
Municipal bonds	1,881	9,875
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	257,090	273,768
Municipal bonds	0	0
Total	257,090	273,768
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	1,881	9,875
Total	1,881	9,875
Available-for-sale marketable securities [Abstract]
Municipal bonds	1,881	9,875
Contractual maturities estimated fair value [Abstract]
Municipal bonds	1,881	9,875
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