CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY Statement - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Treasury Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Loss [Member]
Balance, shares at Dec. 31, 2019		53,999	3,547
Balance, beginning of period at Dec. 31, 2019	$ 2,039,094	$ 541	$ (96,015)	$ 1,508,970	$ 635,246	$ (9,648)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	72,198	0	0	0	72,198	0
Foreign currency translation adjustment	93	0	0	0	0	93
Reclassification of net losses on derivative instruments from OCI to net income, net of tax	226	$ 0	$ 0	0	0	226
Postretirement medical liability adjustment	0
Stock based compensation, shares		146	35
Stock based compensation, value	1,430	$ 0	$ (2,740)	4,170	0	0
Issuance of shares, treasury stock			(11)
Issuance of shares		0
Issuance of shares, value	687	$ 0	$ 317	370	0	0
Dividends on common stock	$ (60,172)	0	0	0	(60,172)	0
Dividends per share	$ 1.200
Balance, end of period at Jun. 30, 2020	$ 2,053,556	$ 541	$ (98,438)	1,513,510	647,272	(9,329)
Balance, shares at Jun. 30, 2020		54,145	3,571
Balance, shares at Mar. 31, 2020		54,145	3,578
Balance, beginning of period at Mar. 31, 2020	2,060,476	$ 541	$ (98,644)	1,512,148	655,865	(9,434)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	21,494	0	0	0	21,494	0
Foreign currency translation adjustment	(8)	0	0	0	0	(8)
Reclassification of net losses on derivative instruments from OCI to net income, net of tax	113	$ 0	$ 0	0	0	113
Postretirement medical liability adjustment	0
Stock based compensation, shares		0	0
Stock based compensation, value	1,139	$ 0	$ 0	1,139	0	0
Stock Issued During Period, Value, Treasury Stock Reissued			$ 206
Issuance of shares, treasury stock			(7)
Issuance of shares		0
Issuance of shares, value	429	$ 0		223	0	0
Dividends on common stock	$ (30,087)	0	$ 0	0	(30,087)	0
Dividends per share	$ 0.60
Balance, end of period at Jun. 30, 2020	$ 2,053,556	$ 541	$ (98,438)	1,513,510	647,272	(9,329)
Balance, shares at Jun. 30, 2020		54,145	3,571
Balance, shares at Dec. 31, 2020		54,145	3,558
Balance, beginning of period at Dec. 31, 2020	2,079,095	$ 541	$ (98,075)	1,513,787	670,111	(7,269)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	100,308	0	0	0	100,308	0
Foreign currency translation adjustment	(55)	0	0	0	0	(55)
Reclassification of net losses on derivative instruments from OCI to net income, net of tax	226	0	0	0	0	226
Postretirement medical liability adjustment	317	$ 0	$ 0	0	0	317
Stock based compensation, shares		93	17
Stock based compensation, value	3,541	$ 1	$ (970)	4,510	0	0
Issuance of shares, treasury stock			(17)
Issuance of shares		880
Issuance of shares, value	57,338	$ 9	$ 467	56,862	0	0
Dividends on common stock	$ (62,821)	0	0	0	(62,821)	0
Dividends per share	$ 1.240
Balance, end of period at Jun. 30, 2021	$ 2,177,315	$ 551	$ (98,578)	1,575,159	707,598	(7,415)
Balance, shares at Jun. 30, 2021		55,118	3,558
Balance, shares at Mar. 31, 2021		54,238	3,563
Balance, beginning of period at Mar. 31, 2021	2,113,835	$ 542	$ (98,730)	1,517,355	702,058	(7,390)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	37,237	0	0	0	37,237	0
Foreign currency translation adjustment	21	0	0	0	0	21
Reclassification of net losses on derivative instruments from OCI to net income, net of tax	113	0	0	0	0	113
Postretirement medical liability adjustment	159	$ 0	$ 0	0	0	159
Stock based compensation, shares		0	0
Stock based compensation, value	1,299	$ 0	$ 0	1,299	0	0
Stock Issued During Period, Value, Treasury Stock Reissued			$ 152
Issuance of shares, treasury stock			(5)
Issuance of shares		880
Issuance of shares, value	56,666	$ 9		56,505	0	0
Dividends on common stock	$ (31,697)	0	$ 0	0	(31,697)	0
Dividends per share	$ 0.620
Balance, end of period at Jun. 30, 2021	$ 2,177,315	$ 551	$ (98,578)	$ 1,575,159	$ 707,598	$ (7,415)
Balance, shares at Jun. 30, 2021		55,118	3,558