Consolidated Statements Of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Cash flows from operating activities:
Net income	$ 44,214	$ 38,156	$ 34,441
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	5,116	4,387	3,764
Bad debt provision	2,160	2,450	2,200
Deferred income tax (benefits)	(3,493)	275	517
Stock-based compensation expense	2,538	2,152	1,332
Amortization of premium on marketable securities	654	999	840
Unrealized loss on marketable securities	82	486	1,083
Unrealized (gain) loss on deferred compensation fund investments	(1,871)	104	(1,325)
Changes in operating assets and liabilities:
Accounts and notes receivable	(11,634)	(24,769)	(6,270)
Prepaid income taxes	405	3,574	(4,014)
Inventories and supplies	(3,531)	(4,531)	(3,639)
Notes receivable — long term	(340)	3,572	(432)
Deferred compensation funding	(4,051)	(1,804)	29
Accounts payable and other accrued expenses	13,664	(1,079)	1,306
Accrued payroll, accrued and withheld payroll taxes	6,142	6,319	4,437
Accrued insurance claims	4,908	732	775
Deferred compensation liability	6,337	2,145	1,697
Income taxes payable	1,906	0	0
Prepaid expenses and other assets	(2,830)	(220)	1,006
Net cash provided by operating activities	60,376	32,948	37,747
Cash flows from investing activities:
Disposals of fixed assets	26	22	44
Additions to property and equipment	(3,484)	(5,545)	(4,174)
Purchases of marketable securities	(10,833)	(18,934)	(38,873)
Sales of marketable securities	19,978	29,971	46,083
Cash paid for acquisition	0	(1,000)	0
Net cash provided by investing activities	5,687	4,514	3,080
Cash flows from financing activities:
Dividends paid	(44,093)	(42,228)	(39,285)
Reissuance of treasury stock pursuant to Dividend Reinvestment Plan	118	128	121
Tax benefit from equity compensation plans	2,649	1,222	1,938
Proceeds from the exercise of stock options	5,573	2,363	4,790
Net cash used in financing activities	(35,753)	(38,515)	(32,436)
Net increase (decrease) in cash and cash equivalents	30,310	(1,053)	8,391
Cash and cash equivalents at beginning of the period	38,639	39,692	31,301
Cash and cash equivalents at end of the period	68,949	38,639	39,692
Supplementary Cash Flow Information:
Issuance of Common Stock in 2012 and 2011, respectively, related to prior year acquisitions	2,633	1,080	0
Issuance of Common Stock in 2012, 2011 and 2010, respectively, pursuant to Employee Stock Plans	$ 1,249	$ 1,233	$ 1,047