CONSOLIDATED STATEMENTS OF COMMON SHAREHOLDERS' EQUITY (USD $) In Thousands, except Per Share data, unless otherwise specified	Total	Common Stock	Paid-in Capital	Treasury Stock	Retained Earnings	Accumulated Other Comprehensive Income
Balance at Dec. 31, 2010	$ 820,347	$ 398	$ 813,878	$ (90,427)	$ 87,984	$ 8,514
Shares, Balance at Dec. 31, 2010		39,799		3,570
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Net Income	92,556	0	0	0	92,556	0
Other comprehensive income:
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	(730)	0	0	0	0	(730)
Reclassification of deferred tax liability on net gains on derivative instruments	(3,572)					(3,572)
Postretirement medical liability adjustment	(581)	0	0	0	0	(581)
Stock based compensation, value	2,669	0	2,762	(93)	0	0
Stock based compensation, shares		42		3
Issuance of shares, value	307	0	60		0	0
Issuance of shares, shares		0
Issuance of shares, value, treasury stock reissued				247
Issuance of shares, shares, treasury stock reissued				(10)
Dividends on common stock	(51,909)	0	0	0	(51,909)	0
Dividends per share	$ 1.44
Balance at Dec. 31, 2011	859,112	398	816,700	(90,273)	128,631	3,656
Shares, Balance at Dec. 31, 2011		39,841		3,563
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)
Reclassification of deferred tax liability on net gains on derivative instruments	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		22
Issuance of shares, value	28,966	9	28,593		0	0
Issuance of shares, shares		815
Issuance of shares, value, treasury stock reissued				364
Issuance of shares, shares, treasury stock reissued				(14)
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		3,571
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)
Reclassification of deferred tax liability on net gains on derivative instruments	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		35
Issuance of shares, value	57,276	14	56,979		0	0
Issuance of shares, shares		1,381
Issuance of shares, value, treasury stock reissued				283
Issuance of shares, shares, treasury stock reissued				(11)
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		3,595