Derivative Instruments, Hedging Activities and Fair Value Measurements, Recurring Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				3 Months Ended										3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013 Liability commodity derivatives - Crude oil [Member] Level 3 [Member]	Mar. 31, 2013 Asset commodity derivatives - Crude oil [Member] Level 3 [Member]	Mar. 31, 2013 Asset commodity derivatives - Crude oil [Member] Liability commodity derivatives - Crude oil [Member] Level 3 [Member]	Mar. 31, 2013 Asset commodity derivatives - Crude oil [Member] Liability commodity derivatives - Crude oil [Member] Level 3 [Member] Minimum [Member]	Mar. 31, 2013 Asset commodity derivatives - Crude oil [Member] Liability commodity derivatives - Crude oil [Member] Level 3 [Member] Maximum [Member]	Mar. 31, 2013 Fair Value [Member] Level 2 [Member]	Dec. 31, 2012 Fair Value [Member] Level 2 [Member]	Mar. 31, 2013 Carrying Value [Member] Level 2 [Member]	Dec. 31, 2012 Carrying Value [Member] Level 2 [Member]	Mar. 31, 2013 Fair Value, Measurements, Recurring [Member]	Mar. 31, 2013 Fair Value, Measurements, Recurring [Member] Level 1 [Member]	Mar. 31, 2013 Fair Value, Measurements, Recurring [Member] Level 2 [Member]	Mar. 31, 2013 Fair Value, Measurements, Recurring [Member] Level 3 [Member]		Mar. 31, 2012 Fair Value, Measurements, Recurring [Member] Level 3 [Member]
Financial assets [Abstract]
Interest rate derivatives												$ 38.4	$ 0	$ 38.4	$ 0
Commodity derivatives				0								52.1	16.3	35.8	0
Financial assets												90.5	16.3	74.2	0
Financial liabilities [Abstract]
Interest rate derivatives												14.7	0	14.7	0
Commodity derivatives			0.6									87.1	39.1	47.4	0.6
Financial liabilities												101.8	39.1	62.1	0.6
Reconciliation of changes in the fair value of Level 3 financial assets and liabilities [Roll Forward]
Balance, beginning of period															(1.5)		0.4
Total gains (losses) included in:
Net income															(0.6)	[1]	0.5	[1]
Other comprehensive income (loss)															0		0.5
Settlements															1.5		(0.5)
Balance, end of period															(0.6)		0.9
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities	(12.3)	15.4													0.9		0.1
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Asset commodity derivatives				0								52.1	16.3	35.8	0
Liability commodity derivatives			0.6									87.1	39.1	47.4	0.6
Fair Value Measurements, Valuation Techniques					Discounted cash flow
Input description					Forward commodity prices
Fair value inputs, forward commodity price (in dollars per unit)						95.75	112.88
Financial Liabilities: [Abstract]
Fixed Rate Debt Principal Amount Fair Value Disclosure								$ 19,540	$ 18,420	$ 17,530	$ 16,180

[1] There were unrealized gains of $0.9 million and $0.1 million included in these amounts for the three months ended March 31, 2013 and 2012, respectively.