Consolidated Statements of Cash Flows - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Cash flows from operating activities:
Net income		$ 22,017	$ 18,626
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization		1,886	1,965
Bad debt provision		1,050	1,100
Deferred income tax expense		0	3,395
Stock-based compensation expense, net of tax benefit from equity compensation plans	[1]	445	609
Amortization of premium on marketable securities		343	318
Unrealized (gain) loss on deferred compensation fund investments		(1,209)	278
Changes in operating assets and liabilities:
Accounts and notes receivable		(10,887)	(22,899)
Inventories and supplies		506	(1,619)
Prepaid expenses and other assets		(2,859)	(3,277)
Notes receivable - long term		475	0
Deferred compensation funding		246	(268)
Accounts payable and other accrued expenses		3,639	(4,100)
Accrued payroll, accrued and withheld payroll taxes		15,299	18,070
Income taxes payable	[1]	1,852	(3,618)
Accrued insurance claims		784	1,009
Deferred compensation liability		1,059	100
Net cash provided by operating activities		34,646	9,689
Cash flows from investing activities:
Disposals of fixed assets		104	114
Additions to property and equipment		(1,325)	(1,459)
Purchases of marketable securities		(7,858)	(4,966)
Sales of marketable securities		7,723	2,640
Net cash used in investing activities		(1,356)	(3,671)
Cash flows from financing activities:
Dividends paid		(13,624)	(13,158)
Reissuance of treasury stock pursuant to Dividend Reinvestment Plan		24	28
Tax benefit from equity compensation plans	[1]	0	509
Proceeds from the exercise of stock options		4,687	2,534
Net cash used in financing activities		(8,913)	(10,087)
Net change in cash and cash equivalents		24,377	(4,069)
Cash and cash equivalents at beginning of the period		23,853	33,189
Cash and cash equivalents at end of the period		48,230	29,120
Supplementary Cash Flow Information:
Cash paid for interest		204	155
Cash paid for income taxes, net of refunds		$ 9,544	$ 11,201

[1] The Company adopted the provisions of ASU 2016-09 prospectively, and as such the amounts reflected for the three months ended March 31, 2016 have not been adjusted.