Consolidated Statements Of Changes In Shareholders' Equity (USD $) In Thousands, unless otherwise specified	Common Stock [Member]	Capital In Excess Of Par Value [Member]	Accumulated Other Comprehensive Income [Member]	Retained Earnings [Member]	Total
Beginning balances at Dec. 31, 2008	$ 9,325	$ 367,000	$ 33,284	$ 1,223,490	$ 1,633,099
Comprehensive income (loss):
Net income (loss)	0	0	0	(55,500)	(55,500)
Other comprehensive income (loss) net of tax:
Change in value of cash flow derivative instruments used as cash flow hedges	0	0	32,307	0	32,307
Reclassification-derivative settlements	0	0	(61,690)	0	(61,690)
Ineffective portion of derivatives qualifying for cash flow hedge accounting	0	0	557	0	557
Total comprehensive income (loss)	0	0	0	0	(84,326)
Activity in employee compensation plans	80	16,957	0	0	17,037
Ending balances at Dec. 31, 2009	9,405	383,957	4,458	1,167,990	1,565,810
Comprehensive income (loss):
Net income (loss)	0	0	0	146,484	146,484
Other comprehensive income (loss) net of tax:
Change in value of cash flow derivative instruments used as cash flow hedges	0	0	21,392	0	21,392
Reclassification-derivative settlements	0	0	(32,268)	0	(32,268)
Ineffective portion of derivatives qualifying for cash flow hedge accounting	0	0	(433)	0	(433)
Total comprehensive income (loss)	0	0	0	0	135,175
Activity in employee compensation plans	88	9,544	0	0	9,632
Ending balances at Dec. 31, 2010	9,493	393,501	(6,851)	1,314,474	1,710,617
Comprehensive income (loss):
Net income (loss)	0	0	0	195,867	195,867
Other comprehensive income (loss) net of tax:
Change in value of cash flow derivative instruments used as cash flow hedges	0	0	29,384	0	29,384
Reclassification-derivative settlements	0	0	(1,819)	0	(1,819)
Ineffective portion of derivatives qualifying for cash flow hedge accounting	0	0	(1,688)	0	(1,688)
Total comprehensive income (loss)	0	0	0	0	221,744
Activity in employee compensation plans	48	14,608	0	0	14,656
Ending balances at Dec. 31, 2011	$ 9,541	$ 408,109	$ 19,026	$ 1,510,341	$ 1,947,017