Derivative Instruments and Hedging Activities - Schedule of Notional Amounts (Details) € in Millions, kWh in Millions, gal in Millions, DTH in Millions, $ in Millions	Dec. 31, 2019 USD ($) DTH gal kWh	Dec. 31, 2019 EUR (€) DTH gal kWh	Sep. 30, 2019 USD ($) DTH gal kWh	Sep. 30, 2019 EUR (€) DTH gal kWh	Dec. 31, 2018 USD ($) DTH gal kWh	Dec. 31, 2018 EUR (€) DTH gal kWh
Commodity contracts | Propane
Derivative [Line Items]
Notional amount (in units) | gal	772.0	772.0	800.4	800.4	450.4	450.4
Commodity contracts | Electricity | Long
Derivative [Line Items]
Notional amount (in units) | kWh	4,145.1	4,145.1	3,098.1	3,098.1	3,974.7	3,974.7
Commodity contracts | Electricity | Short
Derivative [Line Items]
Notional amount (in units) | kWh	555.8	555.8	366.7	366.7	366.7	366.7
Natural gas futures, forward and pipeline contracts (in dekatherms) | Natural Gas
Derivative [Line Items]
Notional amount (in units)	201.4	201.4	196.1	196.1	188.0	188.0
Natural gas basis swap contracts (in dekatherms) | Natural Gas
Derivative [Line Items]
Notional amount (in units)	154.7	154.7	131.1	131.1	81.2	81.2
NYMEX natural gas storage futures contracts (in dekatherms) | Natural Gas
Derivative [Line Items]
Notional amount (in units)	0.4	0.4	0.3	0.3	0.7	0.7
NYMEX natural gas option contracts (in dekatherms) | Natural Gas
Derivative [Line Items]
Notional amount (in units)	2.0	2.0	2.4	2.4	0.0	0.0
NYMEX propane storage futures contracts (in gallons) | Propane
Derivative [Line Items]
Notional amount (in units) | gal	0.1	0.1	0.5	0.5	0.3	0.3
Interest rate swaps
Derivative [Line Items]
Notional amount	$ 1,354.0	€ 300.0	$ 1,357.3	€ 300.0	$ 114.1	€ 300.0
Forward foreign currency exchange contracts
Derivative [Line Items]
Notional amount	$ 431.2	€ 172.8	$ 516.0	€ 172.8	$ 408.6	€ 172.8
Regulated Utility Operations | Commodity contracts | Natural Gas
Derivative [Line Items]
Notional amount (in units)	14.5	14.5	23.3	23.3	14.7	14.7