CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net income	$ 84,402	$ 65,991	$ 39,390
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	18,182	16,644	18,565
Impairment charges and other	0	0	10,480
Amortization of marketable securities	80	90	836
Stock-based compensation	24,345	16,643	13,345
Deferred income taxes	9,742	2,951	(14,733)
Changes in operating assets and liabilities:
Restricted cash	1,500	(5,219)	6,622
Accounts receivable	(63,697)	(69,619)	(25,549)
Prepaid insurance	4,259	(7,624)	13,884
Other current assets	(7,465)	(2,391)	514
Other assets	(12,773)	(8,941)	(22,069)
Accounts payable	2,258	(1,192)	707
Payroll taxes and other payroll deductions payable	55,481	42,373	29,052
Accrued worksite employee payroll expense	52,188	53,297	(30,479)
Accrued health insurance costs	(285)	12,717	(4,686)
Accrued workers’ compensation costs	23,945	21,723	26,159
Accrued corporate payroll, commissions and other accrued liabilities	17,138	3,150	4,105
Income taxes payable/receivable	(4,875)	(7,920)	(1,060)
Total adjustments	120,023	66,682	25,693
Net cash provided by operating activities	204,425	132,673	65,083
Marketable securities:
Purchases	(1,752)	(1,049)	(10,558)
Proceeds from maturities	1,561	1,715	10,593
Proceeds from dispositions	0	7,268	17,869
Property and equipment:
Purchases	(33,337)	(33,994)	(17,844)
Proceeds from sale of aircraft	0	0	12,159
Proceeds from dispositions	278	43	153
Net cash provided by (used in) investing activities	(33,250)	(26,017)	12,372
Cash flows from financing activities:
Purchase of treasury stock	(38,735)	(31,669)	(67,113)
Repurchase of common stock	0	144,263	0
Dividends paid	(65,768)	(20,599)	(21,153)
Borrowings under long-term debt agreement	0	124,400	0
Principal repayments	0	(20,000)	0
Proceeds from the exercise of stock options	0	598	374
Income tax benefit from stock-based compensation	0	0	2,216
Other	1,554	1,373	1,303
Net cash used in financing activities	(102,949)	(90,160)	(84,373)
Net increase (decrease) in cash and cash equivalents	68,226	16,496	(6,918)
Cash and cash equivalents at beginning of year	286,034	269,538	276,456
Cash and cash equivalents at end of year	354,260	286,034	269,538
Supplemental disclosures of cash flow information:
Income taxes, net	40,872	44,148	39,806
Interest expense	$ 3,213	$ 2,396	$ 459