Supplemental Cash Flow Information (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013		Dec. 31, 2012
Supplemental Cash Flow Information [Abstract]
Cash paid for interest, expensed	$ 185,140,000	$ 117,442,000		$ 137,950,000
Cash paid for interest, capitalized	24,202,000	79,253,000		77,432,000
Cash paid for income taxes	5,033,000	28,895,000		99,194,000
Cash received from income tax refunds	(13,193,000)	(17,087,000)		(38,004,000)
Noncash investing activities
Increase in asset retirement obligations	6,500,000	26,946,000		56,290,000
Increase (decrease) in liabilities for capital expenditures	215,000	(18,321,000)		(26,882,000)
Increase in restricted cash	0	0		1,262,559,000	[1]
Decrease in restricted cash	0	1,050,328,000	[2]	212,544,000	[2]
Non-core Gulf Coast Assets [Member]
Business Acquisition [Line Items]
Sales proceeds paid to qualified intermediary		212,500,000
Bakken Exchange Transaction [Member]
Business Acquisition [Line Items]
Sales proceeds paid to qualified intermediary		$ 1,050,000,000

[1] During 2012, $212.5 million of proceeds from the sale of certain non-core assets in the Gulf Coast Region and $1.05 billion of the cash proceeds from the Bakken Exchange Transaction were paid by the respective purchaser directly to a qualified intermediary to facilitate a like-kind-exchange transaction for federal income tax purposes.

[2] During 2012 and 2013, proceeds from the sales of our oil and natural gas property dispositions in 2012, which were held by a qualified intermediary, were released in 2012 to fund the Thompson Field acquisition and in 2013 primarily to fund a portion of the CCA acquisition and certain post-closing costs under the Bakken Exchange Transaction.