Fair Value (Fair Value Of Financial Assets And Liabilities Measured On A Recurring Basis) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Assets
Total	$ 172.1	$ 184.1
Liabilities
Total	101.0	222.8
Fair Value, Inputs, Level 1
Assets
Risk management assets	0.0	0.0
Available-for-sale securities	0.0	0.0
Total	0.0	0.0
Liabilities
Risk management liabilities	0.0	0.0
Total	0.0	0.0
Significant Other Observable Inputs (Level 2)
Assets
Risk management assets	4.2	13.2
Available-for-sale securities	167.9	170.9
Total	172.1	184.1
Liabilities
Risk management liabilities	101.0	222.8
Total	101.0	222.8
Significant Unobservable Inputs (Level 3)
Assets
Risk management assets	0.0	0.0
Available-for-sale securities	0.0	0.0
Total	0.0	0.0
Liabilities
Risk management liabilities	0.0	0.0
Total	0.0	0.0
Available-for-sale Securities
Assets
Available-for-sale securities	167.9	170.9
Risk management assets
Assets
Risk management assets	4.2	13.2
Liabilities
Risk management liabilities	$ 101.0	$ 222.8