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 Income
Balance at Dec. 31, 2013	$ 1,030,670	$ 423	$ 910,184	$ (91,744)	$ 209,091	$ 2,716
Shares, Balance at Dec. 31, 2013		42,340		3,595
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Net Income	120,686	$ 0	0	$ 0	120,686	0
Foreign currency translation adjustment	265	0	0	0	0	265
Reclassification of net gains on derivative instruments from OCI to net income, net of tax	(684)	0	0	0	0	(684)
Realized loss on cash flow hedging derivatives	(11,145)	0	0	0	0	(11,145)
Postretirement medical liability adjustment, net of tax	82	0	0	0	0	82
Stock based compensation, value	3,423	$ 0	4,288	$ (865)	0	0
Stock based compensation, shares		119		12
Issuance of shares, value	399,505	$ 82	399,372		0	0
Issuance of shares, shares		8,063
Issuance of shares, value, treasury stock reissued				$ 51
Issuance of shares, shares, treasury stock reissued				0
Dividends on common stock	$ (65,019)	$ 0	0	$ 0	(65,019)	0
Dividends per share	$ 1.60
Balance at Dec. 31, 2014	$ 1,477,783	$ 505	1,313,844	$ (92,558)	264,758	(8,766)
Shares, Balance at Dec. 31, 2014		50,522		3,607
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Net Income	151,209	$ 0	0	$ 0	151,209	0
Foreign currency translation adjustment	558	0	0	0	0	558
Reclassification of net gains on derivative instruments from OCI to net income, net of tax	(698)	0	0	0	0	(698)
Realized loss on cash flow hedging derivatives	0
Postretirement medical liability adjustment, net of tax	310	0	0	0	0	310
Stock based compensation, value	2,489	$ 0	4,345	$ (1,856)	0	0
Stock based compensation, shares		167		10
Issuance of shares, value	58,581	$ 13	58,102		0	0
Issuance of shares, shares		1,100
Issuance of shares, value, treasury stock reissued				$ 466
Issuance of shares, shares, treasury stock reissued				0
Dividends on common stock	$ (90,058)	$ 0	0	$ 0	(90,058)	0
Dividends per share	$ 1.92
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 gains on derivative instruments from OCI to net income, net of tax	(1,338)	0	0	0	0	(1,338)
Realized loss on cash flow hedging derivatives	0
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