CONSOLIDATED STATEMENTS OF COMMON SHAREHOLDERS' EQUITY (USD $) In Thousands, except Share data, unless otherwise specified	Total	Common Stock	Paid-in Capital	Treasury Stock	Retained Earnings	Accumulated Other Comprehensive Income
Balance at Dec. 31, 2011	$ 859,112	$ 398	$ 816,700	$ (90,273)	$ 128,631	$ 3,656
Shares, Balance at Dec. 31, 2011		39,841,000		3,563,000
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Net Income	98,406	0	0	0	98,406	0
Other comprehensive income:
Foreign currency translation adjustment	(54)	0	0	0	0	(54)
Reclassification of net gains on derivative instruments from OCI to net income, net of tax	(732)	0	0	0	0	(732)
Realized loss on cash flow hedging activities	0
Postretirement medical liability adjustment	(553)	0	0	0	0	(553)
Stock based compensation, value	3,133	1	3,925	(793)	0	0
Stock based compensation, shares		136,000		22,000
Issuance of shares, value	28,966	9	28,593		0	0
Issuance of shares, shares		815,000
Issuance of shares, value, treasury stock reissued				364
Issuance of shares, shares, treasury stock reissued				(14,000)
Dividends on common stock	(54,246)	0	0	0	(54,246)	0
Dividends per share	$ 1.48
Balance at Dec. 31, 2012	934,032	408	849,218	(90,702)	172,791	2,317
Shares, Balance at Dec. 31, 2012		40,792,000		3,571,000
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Net Income	93,983	0	0	0	93,983	0
Other comprehensive income:
Foreign currency translation adjustment	166	0	0	0	0	166
Reclassification of net gains on derivative instruments from OCI to net income, net of tax	(730)	0	0	0	0	(730)
Realized loss on cash flow hedging activities	0
Postretirement medical liability adjustment	963	0	0	0	0	963
Stock based compensation, value	2,663	1	3,987	(1,325)	0	0
Stock based compensation, shares		167,000		35,000
Issuance of shares, value	57,276	14	56,979		0	0
Issuance of shares, shares		1,381,000
Issuance of shares, value, treasury stock reissued				283
Issuance of shares, shares, treasury stock reissued				(11,000)
Dividends on common stock	(57,683)	0	0	0	(57,683)	0
Dividends per share	$ 1.52
Balance at Dec. 31, 2013	1,030,670	423	910,184	(91,744)	209,091	2,716
Shares, Balance at Dec. 31, 2013		42,340,000		3,595,000
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Net Income	120,686	0	0	0	120,686	0
Other comprehensive income:
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 activities	(11,145)					(11,145)
Postretirement medical liability adjustment	82	0	0	0	0	82
Stock based compensation, value	3,423	0	4,288	(865)	0	0
Stock based compensation, shares		119,000		12,000
Issuance of shares, value	399,505	82	399,372		0	0
Issuance of shares, shares		8,063,000
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		50,522,000		3,607,000