Consolidated Statements of Cash Flows - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net income		$ 83,524	$ 88,226	$ 77,396
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization		9,272	8,886	7,496
Bad debt provision		51,387	6,250	4,629
Deferred income tax (benefit) expense		(13,013)	1,887	3,001
Stock-based compensation expense	[1]	5,900	5,985	4,252
Tax benefit from equity compensation plans	[1]	0	0	(2,981)
Amortization of premium on marketable securities		1,373	1,296	1,723
Unrealized loss (gain) on deferred compensation fund investments		1,429	(4,509)	(1,460)
Changes in operating assets and liabilities:
Accounts and notes receivable		(44,363)	(121,639)	(65,610)
Inventories and supplies		950	(1,873)	(1,492)
Prepaid expenses and other assets		1,054	(9,545)	(2,470)
Deferred compensation funding		(1,536)	(257)	2,732
Accounts payable and other accrued expenses		(9,144)	11,197	(4,251)
Accrued payroll, accrued and withheld payroll taxes		6,085	11,927	6,307
Accrued insurance claims		(5,099)	(2,954)	5,404
Deferred compensation liability		450	5,061	(731)
Income taxes payable	[1]	(8,238)	7,692	7,455
Net cash provided by operating activities		80,031	7,630	41,400
Cash flows from investing activities:
Disposals of fixed assets		640	338	275
Additions to property and equipment		(4,940)	(5,397)	(5,442)
Purchases of marketable securities		(14,297)	(33,861)	(29,449)
Sales of marketable securities		9,011	28,537	28,164
Cash paid for acquisitions		0	(4,584)	0
Net cash used in investing activities		(9,586)	(14,967)	(6,452)
Cash flows from financing activities:
Dividends paid		(57,201)	(55,244)	(53,342)
Reissuance of treasury stock pursuant to Dividend Reinvestment Plan		89	95	109
Tax benefit from equity compensation plans		0	0	2,981
Proceeds from the exercise of stock options		8,517	12,808	5,968
Net (repayments) proceeds from short-term borrowings		(5,382)	35,382	0
Net cash used in financing activities		(53,977)	(6,959)	(44,284)
Net change in cash and cash equivalents		16,468	(14,296)	(9,336)
Cash and cash equivalents at beginning of the period		9,557	23,853	33,189
Cash and cash equivalents at end of the period		26,025	9,557	23,853
Supplementary cash flow information:
Cash paid for interest		3,094	1,363	574
Cash paid for income taxes, net of refunds		37,680	35,367	32,532
Contingent shares settled pursuant to acquisition		$ 2,291	$ 0	$ 0

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