Cash, Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014
Overnight holdings
Money market funds (cash equivalents)	$ 187,485	$ 247,720
Investment holdings
Money market funds (cash equivalents)	24,070	26,048
Marketable securities	1,820	9,875
Total cash equivalents and marketable securities	213,375	283,643
Cash held in demand accounts	25,260	19,377
Outstanding checks	(13,265)	(23,607)
Total cash, cash equivalents and marketable securities	225,370	279,413
Cash and cash equivalents	223,550	269,538	$ 201,085	$ 276,456
Marketable securities	1,820	9,875
Withholding associated with federal and state income taxes, employment taxes and other payroll deductions included in cash balance	148,800	185,700
Client prepayments included in cash balance	8,700	17,000
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	211,555	273,768
Municipal bonds	1,820	9,875
Total	213,375	283,643
Available-for-sale marketable securities [Abstract]
Amortized Cost	1,821	9,875
Gross Unrealized Gains	1	3
Gross Unrealized Losses	(2)	(3)
Municipal bonds	1,820	9,875
Contractual maturities amortized cost [Abstract]
Less than one year	1,155
One to five years	666
Amortized Cost	1,821	9,875
Contractual maturities estimated fair value [Abstract]
Less than one year	1,155
One to five years	665
Municipal bonds	1,820	9,875
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	211,555	273,768
Municipal bonds	0	0
Total	211,555	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	1,820	9,875
Total	1,820	9,875
Available-for-sale marketable securities [Abstract]
Municipal bonds	1,820	9,875
Contractual maturities estimated fair value [Abstract]
Municipal bonds	1,820	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