Supplemental Financial Information Exploration and Production (Details 3) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Results of Operations for Oil and Gas Producing Activities, by Geographic Area [Line Items]
Results of Operations, Sales or Transfers to Entity's Other Operations	$ 93,713		$ 115,784		$ 101,230
Results of Operations, Sales Revenue to Unaffiliated Enterprises	359,873		318,565		338,425
Results of Operations, Production or Lifting Costs	140,606		127,403		123,148
Results of Operations, Depreciation, Depletion and Amortization, and Valuation Provisions	139,539	[1]	127,266	[1]	126,278	[1]
Results of Operations, Impairment of Oil and Gas Properties	0		0		620,000
Results of Operations, Income before Income Taxes	173,441		179,680		(429,771)
Income Tax Expense (Benefit)	63,655		66,293		(164,216)
Results of Operations, Oil and Gas Producing Activities Net Income (Excluding Corporate Overhead and Interest Costs)	109,786		113,387		(265,555)
Accretion of discount for asset retirement obligations oil and gas	$ 3,600		$ 3,200		$ 2,700

[1] Includes accretion of discount for asset retirement obligations of $3.6 million, $3.2 million and $2.7 million for the years ended December 31, 2011, 2010 and 2009, respectively, as discussed in Note 10.