Risk Management Activities (Schedule of Derivative Instruments in Statement of Financial Position, Fair Value) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Risk Management Assets Current
Derivatives, Fair Value [Line Items]
Risk management assets	$ 1.0	$ 10.4
Risk Management Assets Noncurrent
Derivatives, Fair Value [Line Items]
Risk management assets	3.2	2.8
Risk Management Liabilities Current
Derivatives, Fair Value [Line Items]
Risk management liabilities	22.1	78.2
Risk Management Liabilities Noncurrent
Derivatives, Fair Value [Line Items]
Risk management liabilities	78.9	144.6
Interest Rate Risk | Risk Management Assets Current
Derivatives, Fair Value [Line Items]
Risk management assets	0.0	0.0
Interest Rate Risk | Risk Management Assets Noncurrent
Derivatives, Fair Value [Line Items]
Risk management assets	0.0	0.0
Interest Rate Risk | Risk Management Liabilities Current
Derivatives, Fair Value [Line Items]
Risk management liabilities	17.1	70.9
Interest Rate Risk | Risk Management Liabilities Noncurrent
Derivatives, Fair Value [Line Items]
Risk management liabilities	40.6	99.5
Commodity Price Risk Programs | Risk Management Assets Current
Derivatives, Fair Value [Line Items]
Risk management assets	1.0	10.4
Commodity Price Risk Programs | Risk Management Assets Noncurrent
Derivatives, Fair Value [Line Items]
Risk management assets	3.2	2.8
Commodity Price Risk Programs | Risk Management Liabilities Current
Derivatives, Fair Value [Line Items]
Risk management liabilities	5.0	7.3
Commodity Price Risk Programs | Risk Management Liabilities Noncurrent
Derivatives, Fair Value [Line Items]
Risk management liabilities	$ 38.3	$ 45.1