CONSOLIDATED STATEMENTS OF COMMON SHAREHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Paid-in Capital	Treasury Stock	Retained Earnings	Accumulated Other Comprehensive Loss
Balance at Dec. 31, 2015	$ 1,600,174	$ 518	$ 1,376,291	$ (93,948)	$ 325,909	$ (8,596)
Shares, Balance at Dec. 31, 2015		51,789		3,617
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Net Income	164,172	$ 0	0	$ 0	164,172	0
Foreign currency translation adjustment	25	0	0	0	0	25
Reclassification of net losses (gains) on derivative instruments from OCI to net income, net of tax	(1,338)	0	0	0	0	(1,338)
Postretirement medical liability adjustment, net of tax	195	0	0	0	0	195
Stock based compensation, value	3,816	$ 0	6,690	$ (2,874)	0	0
Stock based compensation, shares		169		9
Issuance of shares, value	2,345	$ 2	1,290		0	0
Issuance of shares, shares		0
Issuance of shares, value, treasury stock reissued				$ 1,053
Issuance of shares, shares, treasury stock reissued				0
Dividends on common stock	$ (95,765)	$ 0	0	$ 0	(95,765)	0
Dividends per share	$ 2.00
Balance at Dec. 31, 2016	$ 1,676,227	$ 520	1,384,271	$ (95,769)	396,919	(9,714)
Shares, Balance at Dec. 31, 2016		51,958		3,626
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Accounting standard adoption (1)	2,603	$ 0	0	$ 0	2,603	0
Net Income	162,703	0	0	0	162,703	0
Foreign currency translation adjustment	(202)	0	0	0	0	(202)
Reclassification of net losses (gains) on derivative instruments from OCI to net income, net of tax	371	0	0	0	0	371
Postretirement medical liability adjustment, net of tax	773	$ 0	0	0	0	773
Stock based compensation, value	3,541		5,520	$ (1,979)	0	0
Stock based compensation, shares		134		0
Issuance of shares, value	56,772	$ 10	55,390		0	0
Issuance of shares, shares		889
Issuance of shares, value, treasury stock reissued				$ 1,372
Issuance of shares, shares, treasury stock reissued				(17)
Dividends on common stock	$ (101,270)	$ 0	0	$ 0	(101,270)	0
Dividends per share	$ 2.10
Balance at Dec. 31, 2017	$ 1,798,915	$ 530	1,445,181	$ (96,376)	458,352	(8,772)
Shares, Balance at Dec. 31, 2017		52,981		3,609
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Net Income	196,960	$ 0	0	$ 0	196,960	0
Foreign currency translation adjustment	270	0	0	0	0	270
Reclassification of net losses (gains) on derivative instruments from OCI to net income, net of tax	498	0	0	0	0	498
Postretirement medical liability adjustment, net of tax	213	0	0	0	0	213
Stock based compensation, value	6,974	$ 0	7,642	$ (668)	0	0
Stock based compensation, shares		72		12
Issuance of shares, value	47,754	$ 9	46,247		0	0
Issuance of shares, shares		836
Issuance of shares, value, treasury stock reissued				$ 1,498
Issuance of shares, shares, treasury stock reissued				(55)
Dividends on common stock	$ (109,202)	$ 0	0	$ 0	(109,202)	0
Dividends per share	$ 2.20
Balance at Dec. 31, 2018	$ 1,942,382	$ 539	1,499,070	$ (95,546)	548,253	(9,934)
Shares, Balance at Dec. 31, 2018		53,889		3,566
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Reclassification from AOCI to Retained Earnings	$ 0	$ 0	$ 0	$ 0	$ 2,143	$ (2,143)