Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net income	$ 98,682	$ 64,581	$ 83,524
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	14,268	13,940	9,272
Bad debt provision	9,636	25,480	51,387
Deferred income tax benefit	(5,893)	(684)	(13,013)
Share-based compensation expense	7,872	6,865	5,900
Amortization of premium on marketable securities	1,822	1,434	1,373
Unrealized (gain) loss on deferred compensation fund investments	(9,390)	(7,257)	1,429
Loss on sale of assets	1,335	0	0
Changes in operating assets and liabilities:
Accounts and notes receivable	46,158	(29,532)	(44,363)
Inventories and supplies	4,931	4,765	950
Prepaid expenses and other assets	(756)	3,480	1,054
Deferred compensation funding	(52)	(738)	(1,536)
Accounts payable and other accrued expenses	(7,250)	(9,532)	(9,144)
Accrued payroll, accrued and withheld payroll taxes	59,527	3,517	6,085
Income taxes payable	(8,075)	7,845	(5,099)
Accrued insurance claims	(5,194)	8,482	450
Deferred compensation liability	9,592	935	(8,238)
Net cash provided by operating activities	217,213	93,581	80,031
Cash flows from investing activities:
Disposals of fixed assets	274	199	640
Additions to property and equipment	(4,341)	(4,368)	(4,940)
Purchases of marketable securities	(39,047)	(33,544)	(14,297)
Sales of marketable securities	6,269	21,256	9,011
Net cash used in investing activities	(36,845)	(16,457)	(9,586)
Cash flows from financing activities:
Dividends paid	(60,705)	(58,951)	(57,201)
Reissuance of treasury stock pursuant to Dividend Reinvestment Plan	93	90	89
Proceeds from the exercise of stock options	2,920	3,628	8,801
Repayments of short-term borrowings, net	(10,000)	(20,000)	(5,382)
Payments of statutory withholding on net issuance of restricted stock units	(675)	(587)	(284)
Net cash used in financing activities	(68,367)	(75,820)	(53,977)
Net change in cash and cash equivalents	112,001	1,304	16,468
Cash and cash equivalents at beginning of the period	27,329	26,025	9,557
Cash and cash equivalents at end of the period	139,330	27,329	26,025
Supplementary cash flow information:
Cash paid for interest	1,374	3,459	3,094
Cash paid for income taxes, net of refunds	44,865	20,026	37,680
Contingent shares settled pursuant to acquisition	$ 0	$ 1,012	$ 2,291