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		$ 88,226	$ 77,396	$ 58,024
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization		8,886	7,496	7,660
Bad debt provision		6,250	4,629	4,335
Deferred income tax		1,887	3,001	17,842
Stock-based compensation expense, net of tax benefit from equity compensation plans(1)	[1]	276	1,271	1,668
Amortization of premium on marketable securities		1,296	1,723	681
Unrealized (gain) loss on deferred compensation fund investments		(4,509)	(1,460)	24
Changes in operating assets and liabilities:
Accounts and notes receivable		(121,639)	(65,610)	(18,854)
Inventories and supplies		(1,873)	(1,492)	(846)
Prepaid expenses and other assets		(9,545)	(2,470)	(1,710)
Deferred compensation funding		(257)	2,732	(649)
Accounts payable and other accrued expenses		11,197	(4,251)	2,403
Accrued payroll, accrued and withheld payroll taxes		11,927	6,307	(28,314)
Accrued insurance claims		(2,954)	5,404	13,987
Deferred compensation liability		5,061	(731)	1,113
Income taxes payable	[1]	13,401	7,455	5,997
Net cash provided by operating activities		7,630	41,400	63,361
Cash flows from investing activities:
Disposals of fixed assets		338	275	267
Additions to property and equipment		(5,397)	(5,442)	(4,998)
Purchases of marketable securities		(33,861)	(29,449)	(75,150)
Sales of marketable securities		28,537	28,164	17,567
Cash paid for acquisitions		(4,584)	0	0
Net cash used in investing activities		(14,967)	(6,452)	(62,314)
Cash flows from financing activities:
Dividends paid		(55,244)	(53,342)	(51,375)
Reissuance of treasury stock pursuant to Dividend Reinvestment Plan		95	109	113
Tax benefit from equity compensation plans	[1]	0	2,981	1,873
Proceeds from the exercise of stock options		12,808	5,968	6,251
Net proceeds from short-term borrowings		35,382	0	0
Net cash used in financing activities		(6,959)	(44,284)	(43,138)
Net change in cash and cash equivalents		(14,296)	(9,336)	(42,091)
Cash and cash equivalents at beginning of the period		23,853	33,189	75,280
Cash and cash equivalents at end of the period		9,557	23,853	33,189
Supplementary cash flow information:
Cash paid for interest		1,363	574	258
Cash paid for income taxes, net of refunds		$ 35,367	$ 32,532	$ 7,901

[1] The Company adopted the provisions of ASU 2016-09 prospectively, and as such the amounts reflected for the years ended December 31, 2016 and 2015 have not been adjusted.