Restatement of Previously Issued Condensed Consolidated Financial Statements - Impact on Condensed Consolidated Statement of Cash Flows (Unaudited) (Detail) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015		Jun. 30, 2016	Jun. 30, 2015
Cash flows from operating activities:
Net income	$ 8,522	$ 8,902	[1]	$ 519	$ 3,074	[1]
Reconciliations of net income to net cash provided by operating activities:
Depreciation and amortization	769	709	[1]	1,518	1,392	[1]
Losses recognized on marketable securities				3	1	[1]
Deferred income taxes				0	72	[1]
Share based compensation				1,040	1,069	[1]
Excess tax benefit from share-based compensation				(265)	(70)	[1]
Changes in certain operating assets and liabilities:
Trade accounts receivable				(87,398)	(35,235)	[1]
Income taxes receivable				1,038	2,753	[1]
Prepaid expenses and other				(1,645)	(1,876)	[1]
Accounts payable				1,241	(300)	[1]
Accrued payroll, payroll taxes and related benefits				64,952	24,919	[1]
Other accrued liabilities				(1,747)	(1,385)	[1]
Workers' compensation claims liabilities				21,375	13,966	[1]
Safety incentives liability				3,009	3,915	[1]
Customer deposits, long-term liabilities and other assets, net				495	278	[1]
Net cash provided by operating activities				4,471	12,573	[1]
Cash flows from investing activities:
Purchase of property and equipment				(2,656)	(1,365)	[1]
Purchase of marketable securities				(284)	(2,315)	[1]
Proceeds from sales and maturities of marketable securities				4,026	47,197	[1]
Purchase of restricted marketable securities				(79,441)	(64,483)	[1]
Proceeds from maturities of restricted marketable securities				59,574	3,621	[1]
Net cash used in investing activities				(18,781)	(17,345)	[1]
Cash flows from financing activities:
Proceeds from credit-line borrowings				11,300	44,934	[1]
Payments on credit-line borrowings				(11,300)	(41,711)	[1]
Payments on long-term debt				(7,610)	(3,110)	[1]
Common stock repurchased on vesting of restricted stock units				(40)	(65)	[1]
Dividends paid				(3,173)	(3,141)	[1]
Proceeds from the exercise of stock options and vesting of restricted stock units				72	690	[1]
Excess tax benefit from share-based compensation				265	70	[1]
Net cash used in financing activities				(10,486)	(2,333)	[1]
Net decrease in cash and cash equivalents				(24,796)	(7,105)	[1]
Cash and cash equivalents, beginning of period				25,218	11,544	[1]
Cash and cash equivalents, end of period	$ 422	4,439	[1]	$ 422	4,439	[1]
As Previously Reported [Member]
Cash flows from operating activities:
Net income		8,746			2,944
Reconciliations of net income to net cash provided by operating activities:
Depreciation and amortization		709			1,392
Losses recognized on marketable securities					1
Deferred income taxes					0
Share based compensation					1,069
Excess tax benefit from share-based compensation					(70)
Changes in certain operating assets and liabilities:
Trade accounts receivable					(35,235)
Income taxes receivable					2,754
Prepaid expenses and other					(1,876)
Accounts payable					(300)
Accrued payroll, payroll taxes and related benefits					24,094
Other accrued liabilities					(21)
Workers' compensation claims liabilities					13,629
Safety incentives liability					3,915
Customer deposits, long-term liabilities and other assets, net					278
Net cash provided by operating activities					12,574
Cash flows from investing activities:
Purchase of property and equipment					(1,365)
Purchase of marketable securities					(2,315)
Proceeds from sales and maturities of marketable securities					47,197
Purchase of restricted marketable securities					(64,483)
Proceeds from maturities of restricted marketable securities					3,621
Net cash used in investing activities					(17,345)
Cash flows from financing activities:
Proceeds from credit-line borrowings					44,934
Payments on credit-line borrowings					(41,711)
Payments on long-term debt					(3,110)
Common stock repurchased on vesting of restricted stock units					(65)
Dividends paid					(3,142)
Proceeds from the exercise of stock options and vesting of restricted stock units					690
Excess tax benefit from share-based compensation					70
Net cash used in financing activities					(2,334)
Net decrease in cash and cash equivalents					(7,105)
Cash and cash equivalents, beginning of period					11,544
Cash and cash equivalents, end of period		4,439			4,439
Restatement Adjustments [Member]
Cash flows from operating activities:
Net income		156			130
Reconciliations of net income to net cash provided by operating activities:
Depreciation and amortization		0			0
Losses recognized on marketable securities					0
Deferred income taxes					72
Share based compensation					0
Excess tax benefit from share-based compensation					0
Changes in certain operating assets and liabilities:
Trade accounts receivable					0
Income taxes receivable					(1)
Prepaid expenses and other					0
Accounts payable					0
Accrued payroll, payroll taxes and related benefits					825
Other accrued liabilities					(1,364)
Workers' compensation claims liabilities					337
Safety incentives liability					0
Customer deposits, long-term liabilities and other assets, net					0
Net cash provided by operating activities					(1)
Cash flows from investing activities:
Purchase of property and equipment					0
Purchase of marketable securities					0
Proceeds from sales and maturities of marketable securities					0
Purchase of restricted marketable securities					0
Proceeds from maturities of restricted marketable securities					0
Net cash used in investing activities					0
Cash flows from financing activities:
Proceeds from credit-line borrowings					0
Payments on credit-line borrowings					0
Payments on long-term debt					0
Common stock repurchased on vesting of restricted stock units					0
Dividends paid					1
Proceeds from the exercise of stock options and vesting of restricted stock units					0
Excess tax benefit from share-based compensation					0
Net cash used in financing activities					1
Net decrease in cash and cash equivalents					0
Cash and cash equivalents, beginning of period					0
Cash and cash equivalents, end of period		$ 0			$ 0

[1] See Note 2. Restatement of Previously Issued Condensed Consolidated Financial Statements