Derivative Instruments, Hedging Activities and Fair Value Measurements, Recurring Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Total gains (losses) included in:
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities	$ 29.5		$ 25.7		$ (21.6)
Liability commodity derivatives - Crude oil [Member] | Level 3 [Member]
Financial liabilities [Abstract]
Commodity derivatives	3.9
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Liability commodity derivatives	3.9
Asset commodity derivatives - Crude oil [Member] | Level 3 [Member]
Financial assets [Abstract]
Commodity derivatives	2.4
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Asset commodity derivatives	2.4
Asset commodity derivatives - Crude oil [Member] | Liability commodity derivatives - Crude oil [Member] | Level 3 [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Fair Value Measurements, Valuation Techniques	Discounted cash flow
Input description	Forward commodity prices
Asset commodity derivatives - Crude oil [Member] | Liability commodity derivatives - Crude oil [Member] | Level 3 [Member] | Minimum [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Fair value inputs, forward commodity price (in dollars per unit)	75.62
Asset commodity derivatives - Crude oil [Member] | Liability commodity derivatives - Crude oil [Member] | Level 3 [Member] | Maximum [Member]
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Fair value inputs, forward commodity price (in dollars per unit)	92.28
Level 3 [Member]
Reconciliation of changes in the fair value of Level 3 financial assets and liabilities [Roll Forward]
Balance, beginning of period	0.4		(25.9)
Total gains (losses) included in:
Net income	(2.9)	[1]	2.3	[1]
Other comprehensive income (loss)	10.1		16.2
Settlements	0.8		(2.0)
Transfers out of Level 3	(9.9)	[2]	9.8	[2]
Balance, end of period	(1.5)		0.4
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities	(1.9)		2.6
Fair Value [Member] | Level 2 [Member]
Financial Liabilities: [Abstract]
Fixed Rate Debt Principal Amount Fair Value Disclosure	18,420		15,760
Carrying Value [Member]
Financial Liabilities: [Abstract]
Fixed Rate Debt Principal Amount Fair Value Disclosure	16,180		14,330
Fair Value, Measurements, Recurring [Member]
Financial assets [Abstract]
Interest rate derivatives	45.2		87.9
Commodity derivatives	61.6		67.3
Financial assets	106.8		155.2
Financial liabilities [Abstract]
Interest rate derivatives	192.6		311.4
Commodity derivatives	45.5		65.0
Financial liabilities	238.1		376.4
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Asset commodity derivatives	61.6		67.3
Liability commodity derivatives	45.5		65.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Financial assets [Abstract]
Interest rate derivatives	0		0
Commodity derivatives	11.4		28.4
Financial assets	11.4		28.4
Financial liabilities [Abstract]
Interest rate derivatives	0		0
Commodity derivatives	13.1		29.9
Financial liabilities	13.1		29.9
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Asset commodity derivatives	11.4		28.4
Liability commodity derivatives	13.1		29.9
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Financial assets [Abstract]
Interest rate derivatives	45.2		87.9
Commodity derivatives	47.8		38.1
Financial assets	93.0		126.0
Financial liabilities [Abstract]
Interest rate derivatives	192.6		311.4
Commodity derivatives	28.5		34.7
Financial liabilities	221.1		346.1
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Asset commodity derivatives	47.8		38.1
Liability commodity derivatives	28.5		34.7
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Financial assets [Abstract]
Interest rate derivatives	0		0
Commodity derivatives	2.4		0.8
Financial assets	2.4		0.8
Financial liabilities [Abstract]
Interest rate derivatives	0		0
Commodity derivatives	3.9		0.4
Financial liabilities	3.9		0.4
Fair Value Measurements, Recurring, Valuation Techniques [Line Items]
Asset commodity derivatives	2.4		0.8
Liability commodity derivatives	$ 3.9		$ 0.4

[1] There were $1.9 million of unrealized losses and $2.6 million of unrealized gains included in these amounts for the years ended December 31, 2012 and 2011, respectively.

[2] Transfers out of Level 3 into Level 2 during 2012 were due to shorter remaining transaction maturities falling inside of the Level 2 range during the fourth quarter. Transfers out of Level 3 into Level 2 during 2011 were primarily due to the change in observability of forward NGL prices.