Fair Value (Fair Value Of Financial Assets And Liabilities Measured On A Recurring Basis) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Risk management assets	$ 49.0	$ 0.1
Available-for-sale securities	131.5	128.7
Fair value assets measured on recurring basis, total	180.5	128.8
Risk management liabilities	61.3	31.9
Fair value liabilities measured on recurring basis, total	61.3	31.9
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Risk management assets	5.4	0.1
Available-for-sale securities	0.0	0.0
Fair value assets measured on recurring basis, total	5.4	0.1
Risk management liabilities	1.2	14.3
Fair value liabilities measured on recurring basis, total	1.2	14.3
Significant Other Observable Inputs (Level 2) [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Risk management assets	43.6	0.0
Available-for-sale securities	131.5	128.7
Fair value assets measured on recurring basis, total	175.1	128.7
Risk management liabilities	58.9	17.4
Fair value liabilities measured on recurring basis, total	58.9	17.4
Significant Unobservable Inputs (Level 3) [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Risk management assets	0.0	0.0
Available-for-sale securities	0.0	0.0
Fair value assets measured on recurring basis, total	0.0	0.0
Risk management liabilities	1.2	0.2
Fair value liabilities measured on recurring basis, total	$ 1.2	$ 0.2