Fair Value Disclosures (Fair Value Of Financial Assets And Liabilities Measured On A Recurring Basis) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sale securities	$ 108.6	$ 111.8
Fair value assets measured on recurring basis, total	182.1	260.0
Fair value liabilities measured on recurring basis, total	52.5	115.5
Physical Price Risk Programs [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value assets measured on recurring basis, derivative financial instruments	14.6	35.4
Fair value liabilities measured on recurring basis, total	0	0
Financial Price Risk Programs [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value assets measured on recurring basis, derivative financial instruments	28.0	72.4
Fair value liabilities measured on recurring basis, total	52.5	115.5
Interest Rate Risk Activities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value assets measured on recurring basis, derivative financial instruments	30.9	40.4
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sale securities	22.4	27.4
Fair value assets measured on recurring basis, total	50.0	98.9
Fair value liabilities measured on recurring basis, total	51.3	115.0
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Physical Price Risk Programs [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value assets measured on recurring basis, derivative financial instruments	0	0
Fair value liabilities measured on recurring basis, derivative financial instruments	0	0
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Financial Price Risk Programs [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value assets measured on recurring basis, derivative financial instruments	27.6	71.5
Fair value liabilities measured on recurring basis, derivative financial instruments	51.3	115.0
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Interest Rate Risk Activities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value assets measured on recurring basis, derivative financial instruments	0	0
Significant Other Observable Inputs (Level 2) [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sale securities	86.2	84.4
Fair value assets measured on recurring basis, total	132.1	161.0
Fair value liabilities measured on recurring basis, total	0.2	0.5
Significant Other Observable Inputs (Level 2) [Member] | Physical Price Risk Programs [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value assets measured on recurring basis, derivative financial instruments	14.6	35.4
Fair value liabilities measured on recurring basis, derivative financial instruments	0	0
Significant Other Observable Inputs (Level 2) [Member] | Financial Price Risk Programs [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value assets measured on recurring basis, derivative financial instruments	0.4	0.8
Fair value liabilities measured on recurring basis, derivative financial instruments	0.2	0.5
Significant Other Observable Inputs (Level 2) [Member] | Interest Rate Risk Activities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value assets measured on recurring basis, derivative financial instruments	30.9	40.4
Significant Unobservable Inputs (Level 3) [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sale securities	0	0
Fair value assets measured on recurring basis, total	0	0.1
Fair value liabilities measured on recurring basis, total	1.0	0
Significant Unobservable Inputs (Level 3) [Member] | Physical Price Risk Programs [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value assets measured on recurring basis, derivative financial instruments	0	0
Fair value liabilities measured on recurring basis, derivative financial instruments	0	0
Significant Unobservable Inputs (Level 3) [Member] | Financial Price Risk Programs [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value assets measured on recurring basis, derivative financial instruments	0	0.1
Fair value liabilities measured on recurring basis, derivative financial instruments	1.0	0
Significant Unobservable Inputs (Level 3) [Member] | Interest Rate Risk Activities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value assets measured on recurring basis, derivative financial instruments	$ 0	$ 0