Fair Value Measurements - Fair Value of Financial Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Financial Assets
Derivative Asset, Fair Value, Gross	$ 6.0	$ 6.5
Financial liabilities
Derivative Liability, Fair Value, Gross	129.7	32.0
Derivative instruments - long-term	16.1	0
Fair Value, Measurements, Recurring [Member]
Financial Assets
Total financial assets	1.7	1.2
Financial liabilities
Total financial liabilities	125.4	26.7
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member]
Financial Assets
Total financial assets	0	0
Financial liabilities
Total financial liabilities	0	0
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Commodity Contract [Member]
Financial Assets
Derivative instruments - short-term		0
Derivative instruments - long-term		0
Financial liabilities
Derivative instruments - short-term		0
Derivative instruments - long-term		0
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Interest Rate Swap [Member]
Financial liabilities
Derivative instruments - short-term		0
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member]
Financial Assets
Total financial assets	6.0	6.5
Financial liabilities
Total financial liabilities	129.7	32.0
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member]
Financial Assets
Total financial assets	0	0
Financial liabilities
Total financial liabilities	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Commodity Contract [Member]
Financial Assets
Derivative instruments - short-term		0
Derivative instruments - long-term		0
Financial liabilities
Derivative instruments - short-term		0
Derivative instruments - long-term		0
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Interest Rate Swap [Member]
Financial liabilities
Derivative instruments - short-term		0
Fair Value, Measurements, Recurring [Member] \| Netting Adjustments [Member]
Financial Assets
Total financial assets	(4.3)	(5.3)
Financial liabilities
Total financial liabilities	(4.3)	(5.3)
Long-term [Member] \| Fair Value, Measurements, Recurring [Member] \| Commodity Contract [Member]
Financial Assets
Derivative instruments - long-term	0.5	0.4
Financial liabilities
Derivative instruments - long-term	16.1
Long-term [Member] \| Fair Value, Measurements, Recurring [Member] \| Interest Rate Swap [Member]
Financial Assets
Derivative instruments - long-term	1.2	0.6
Long-term [Member] \| Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Commodity Contract [Member]
Financial Assets
Derivative instruments - long-term	0
Financial liabilities
Derivative instruments - long-term	0
Long-term [Member] \| Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Interest Rate Swap [Member]
Financial Assets
Derivative instruments - long-term	0
Long-term [Member] \| Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Commodity Contract [Member]
Financial Assets
Derivative instruments - long-term	0
Financial liabilities
Derivative instruments - long-term	0
Long-term [Member] \| Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Interest Rate Swap [Member]
Financial Assets
Derivative instruments - long-term	0
Long-term [Member] \| Fair Value, Measurements, Recurring [Member] \| Netting Adjustments [Member] \| Commodity Contract [Member]
Financial Assets
Derivative instruments - long-term	0	0
Financial liabilities
Derivative instruments - long-term	0
Long-term [Member] \| Fair Value, Measurements, Recurring [Member] \| Netting Adjustments [Member] \| Interest Rate Swap [Member]
Financial Assets
Derivative instruments - long-term	0	0
Long-term [Member] \| Interest Rate Swap [Member]
Financial Assets
Derivative Asset, Fair Value, Gross	1.2	0.6
Financial liabilities
Derivative Liability, Fair Value, Gross	0	0
Long-term [Member] \| Interest Rate Swap [Member] \| Fair Value, Measurements, Recurring [Member] \| Level 2 [Member]
Financial Assets
Derivative Asset, Fair Value, Gross		0.6
Long-term [Member] \| Commodity Contract [Member]
Financial Assets
Derivative Asset, Fair Value, Gross	0.5	0.4
Financial liabilities
Derivative Liability, Fair Value, Gross	16.1	0
Long-term [Member] \| Commodity Contract [Member] \| Fair Value, Measurements, Recurring [Member] \| Level 2 [Member]
Financial Assets
Derivative Asset, Fair Value, Gross		0.4
Short-term [Member] \| Fair Value, Measurements, Recurring [Member] \| Commodity Contract [Member]
Financial Assets
Derivative instruments - short-term	0	0.2
Financial liabilities
Derivative instruments - short-term	104.6	24.1
Short-term [Member] \| Fair Value, Measurements, Recurring [Member] \| Interest Rate Swap [Member]
Financial liabilities
Derivative instruments - short-term	4.7	2.6
Short-term [Member] \| Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Commodity Contract [Member]
Financial Assets
Derivative instruments - short-term	0
Financial liabilities
Derivative instruments - short-term	0
Short-term [Member] \| Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Interest Rate Swap [Member]
Financial liabilities
Derivative instruments - short-term	0
Short-term [Member] \| Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Commodity Contract [Member]
Financial liabilities
Derivative Liability, Fair Value, Gross		29.4
Short-term [Member] \| Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Interest Rate Swap [Member]
Financial liabilities
Derivative Liability, Fair Value, Gross		2.6
Short-term [Member] \| Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Commodity Contract [Member]
Financial Assets
Derivative instruments - short-term	0
Financial liabilities
Derivative instruments - short-term	0
Short-term [Member] \| Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Interest Rate Swap [Member]
Financial liabilities
Derivative instruments - short-term	0
Short-term [Member] \| Fair Value, Measurements, Recurring [Member] \| Netting Adjustments [Member] \| Commodity Contract [Member]
Financial Assets
Derivative instruments - short-term	(4.3)	(5.3)
Financial liabilities
Derivative instruments - short-term	(4.3)	(5.3)
Short-term [Member] \| Fair Value, Measurements, Recurring [Member] \| Netting Adjustments [Member] \| Interest Rate Swap [Member]
Financial liabilities
Derivative instruments - short-term	0	0
Short-term [Member] \| Interest Rate Swap [Member]
Financial Assets
Derivative Asset, Fair Value, Gross	0	0
Financial liabilities
Derivative Liability, Fair Value, Gross	4.7	2.6
Short-term [Member] \| Commodity Contract [Member]
Financial Assets
Derivative Asset, Fair Value, Gross	4.3	5.5
Financial liabilities
Derivative Liability, Fair Value, Gross	108.9	29.4
Short-term [Member] \| Commodity Contract [Member] \| Fair Value, Measurements, Recurring [Member] \| Level 2 [Member]
Financial Assets
Derivative Asset, Fair Value, Gross		$ 5.5

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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