Earnings Per Share (Schedule Of Earnings Per Share) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Earnings Per Share [Abstract]
Income of basic earnings per common share	$ (56,547)		$ 46,586		$ (19,302)		$ 52,439		$ 51,661		$ 53,360		$ 49,819		$ 41,027		$ 23,176	$ 195,867	$ 146,484
Income of effect of dilutive stock options, restricted stock and stock appreciation rights (SARs)																	0	0	0
Income of diluted earnings per common share																	$ 23,176	$ 195,867	$ 146,484
Weighted shares of basic earnings per common share																	47,909	47,658	47,278
Weighted shares of effect of dilutive stock options, restricted stock and stock appreciation rights (SARs)																	245	293	176
Weighted shares of diluted earnings per common share																	48,154	47,951	47,454
Per share amount of basic earnings per common share	$ (1.18)	[1]	$ 0.97	[1]	$ (0.40)	[1]	$ 1.10	[1]	$ 1.08	[2]	$ 1.12	[2]	$ 1.05	[2]	$ 0.86	[2]	$ 0.48	$ 4.11	$ 3.10
Per share amount of effect of dilutive stock options, restricted stock and stock appreciation rights (SARs)																	$ 0.00	$ (0.03)	$ (0.01)
Per share amount of diluted earnings per common share	$ (1.18)		$ 0.97		$ (0.40)		$ 1.09		$ 1.08		$ 1.11		$ 1.04		$ 0.86		$ 0.48	$ 4.08	$ 3.09

[1] Plugging liability estimates were revised in March 2012 for updates in the cost of services used to plug wells over the preceding year. Although cost per well increased, a slight decrease in the inflation factor resulted in a decrease in estimated cost. Costs were reviewed each quarter resulting in no change to the March 2012 estimates.

[2] Due to the effect of rounding the basic earnings or diluted per share for the year’s four quarters does not equal annual earnings per share.