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	$ 25,170	$ 18,799	$ 25,494
Reconciliations of net income to net cash from operating activities:
Depreciation and amortization	5,452	3,253	2,851
Gains recognized on investments	(51)	(3)	(2)
Losses recognized on sale of property		31
Deferred income taxes	4,039	(1,704)	2,728
Share-based compensation	4,184	2,782	2,386
Excess tax from share-based compensation		(253)	68
Changes in certain operating assets and liabilities:
Trade accounts receivable	(10,180)	(35,955)	12,097
Income taxes receivable	(1,686)	1,038	10,521
Prepaid expenses and other	(1,825)	(726)	640
Accounts payable	222	1,727	498
Accrued payroll, payroll taxes and related benefits	28,529	31,767	5,506
Other accrued liabilities	1,350	1,508	(64)
Income taxes payable	(3,041)	3,294	506
Workers' compensation claims liabilities	57,140	51,235	30,397
Safety incentives liability	3,697	3,582	7,021
Customer deposits, long-term liabilities and other assets, net	(141)	(68)	(16)
Net cash provided by operating activities	112,859	80,307	100,631
Cash flows from investing activities:
Purchase of property and equipment	(3,687)	(7,106)	(2,996)
Proceeds from sale of property		1,459
Purchase of investments	(6,283)	(264)	(8,214)
Proceeds from sales and maturities of investments	10,718	4,796	52,996
Purchase of restricted cash and investments	(1,078,309)	(185,845)	(336,083)
Proceeds from sales and maturities of restricted cash and investments	982,776	153,890	238,701
Net cash used in investing activities	(94,785)	(33,070)	(55,596)
Cash flows from financing activities:
Proceeds from credit-line borrowings	24,899	14,868	46,106
Payments on credit-line borrowings	(24,899)	(14,868)	(46,106)
Payments on long-term debt	(221)	(15,220)	(25,220)
Common stock repurchased on vesting of restricted stock units	(1,673)	(433)	(465)
Dividends paid	(7,276)	(6,359)	(6,310)
Proceeds from exercise of stock options	163	72	702
Excess tax benefits from share-based compensation		253	(68)
Net cash used in financing activities	(9,007)	(21,687)	(31,361)
Net increase in cash and cash equivalents	9,067	25,550	13,674
Cash and cash equivalents, beginning of period	50,768	25,218	11,544
Cash and cash equivalents, end of period	$ 59,835	$ 50,768	$ 25,218