Supplemental Gas and Oil Information (Unaudited) Change in Standardized Measure of Future Cash Flows (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Change in Standardized Measure of Future Cash Flows [Abstract]
Beginning Balance	$ 3,525.6	$ 2,705.6	$ 1,443.0
Sales of gas, oil and NGL produced during the period, net of production costs	(892.3)	(1,779.9)	(1,125.7)
Net change in sales prices and in production (lifting) costs related to future production	(2,083.5)	1,472.5	1,775.8
Net change due to extensions, discoveries and improved recovery	948.5	1,806.4	789.1
Net change due to revisions of quantity estimates	(387.8)	(48.2)	140.4
Changes due to purchases of reserves in place	831.4	0	0
Changes due to sales of reserves in place	0	(8.0)	(26.0)
Previously estimated development costs incurred during the period	513.0	533.6	434.2
Changes in estimated future development costs	(209.3)	(1,110.4)	(325.4)
Accretion of discount	499.4	355.4	170.9
Net change in income taxes	273.6	(411.4)	(582.4)
Other	16.1	9.9	11.5
Net change	(490.9)	820.0	1,262.6
Ending Balance	$ 3,034.7	$ 3,525.6	$ 2,705.6

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The increase in the net discounted value of the proved oil and gas reserves due only to the passage of time.

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The change in the discounted value of the proved oil and gas reserves due to financial transactions where reserves (minerals in place) were sold.

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This is the amount that proved oil and gas reserves increased due to extensions (increased estimates in the size of proven reserves), discoveries, additions, and improved recovery (increase in ability to extract oil and gas from existing reserves).

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The increase (decrease) in the evaluation or calculation of the costs that will be required in future periods to make it possible to extract or process proved oil and gas reserves.

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The increase (decrease) in the estimate of future income taxes that will be due on future pretax net cash flows relating to the enterprise's proved oil and gas reserves.

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The change in the discounted value of the proved oil and gas reserves due to financial transactions where new reserves were purchased.

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The increase (decrease) in sales and transfer prices and in production (lifting) costs related to future production. The effects of changes in prices and costs are computed before the effects of changes in quantities.

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The actual costs incurred during the reporting period to make it possible to extract or process proved oil and gas reserves. These costs were previously estimated and reduced the estimate of discounted future cash flows from oil and gas reserves.

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Any changes made during the period to the evaluation or calculation of the estimated quantity of oil or gas in proved reserves.

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The amount of sales or transfers of oil gas that was previously included in proved oil and gas reserves. This amount is net of production costs.

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Changes in the standardized measure of discounted future net cash flows relating to proved oil and gas reserves that are not otherwise specified in the taxonomy.

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The increase (decrease) as a result of changes in the elements used to estimate the discounted future net cash flow relating to proved oil and gas reserves.

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The balance as of the balance sheet date of the standardized measure of discounted future net cash flow relating to proved oil and gas reserves.

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