Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net income	$ 64,581	$ 83,524	$ 88,226
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	13,940	9,272	8,886
Bad debt provision	25,480	51,387	6,250
Deferred income tax (benefit) expense	(684)	(13,013)	1,887
Stock-based compensation expense	6,865	5,900	5,985
Amortization of premium on marketable securities	1,434	1,373	1,296
Unrealized (gain) loss on deferred compensation fund investments	(7,257)	1,429	(4,509)
Changes in operating assets and liabilities:
Accounts and notes receivable	(29,532)	(44,363)	(121,639)
Inventories and supplies	4,765	950	(1,873)
Prepaid expenses and other assets	3,480	1,054	(9,545)
Deferred compensation funding	(738)	(1,536)	(257)
Accounts payable and other accrued expenses	(9,532)	(9,144)	11,197
Accrued payroll, accrued and withheld payroll taxes	3,517	6,085	11,927
Accrued insurance claims	7,845	(5,099)	(2,954)
Deferred compensation liability	8,482	450	5,061
Income taxes payable	935	(8,238)	7,692
Net cash provided by operating activities	93,581	80,031	7,630
Cash flows from investing activities:
Disposals of fixed assets	199	640	338
Additions to property and equipment	(4,368)	(4,940)	(5,397)
Purchases of marketable securities	(33,544)	(14,297)	(33,861)
Sales of marketable securities	21,256	9,011	28,537
Cash paid for acquisitions	0	0	(4,584)
Net cash used in investing activities	(16,457)	(9,586)	(14,967)
Cash flows from financing activities:
Dividends paid	(58,951)	(57,201)	(55,244)
Reissuance of treasury stock pursuant to Dividend Reinvestment Plan	90	89	95
Proceeds from the exercise of stock options	3,628	8,801	12,808
Net (repayments) proceeds from short-term borrowings	(20,000)	(5,382)	35,382
Payments of statutory withholding on net issuance of restricted stock units	(587)	(284)	0
Net cash used in financing activities	(75,820)	(53,977)	(6,959)
Net change in cash and cash equivalents	1,304	16,468	(14,296)
Cash and cash equivalents at beginning of the period	26,025	9,557	23,853
Cash and cash equivalents at end of the period	27,329	26,025	9,557
Supplementary cash flow information:
Cash paid for interest	3,459	3,094	1,363
Cash paid for income taxes, net of refunds	20,026	37,680	35,367
Contingent shares settled pursuant to acquisition	$ 1,012	$ 2,291	$ 0