Derivative Instruments and Hedging Activities - Schedule of Notional Amounts (Details) € in Millions, kWh in Millions, gal in Millions, DTH in Millions, $ in Millions	Mar. 31, 2020 USD ($) DTH gal kWh	Mar. 31, 2020 EUR (€) DTH gal kWh	Sep. 30, 2019 USD ($) DTH gal kWh	Sep. 30, 2019 EUR (€) DTH gal kWh	Mar. 31, 2019 USD ($) DTH gal kWh	Mar. 31, 2019 EUR (€) DTH gal kWh
Commodity contracts | Propane
Derivative [Line Items]
Notional amount (in units) | gal	816.4	816.4	800.4	800.4	369.2	369.2
Commodity contracts | Electricity | Long
Derivative [Line Items]
Notional amount (in units) | kWh	4,165.5	4,165.5	3,098.1	3,098.1	3,685.0	3,685.0
Commodity contracts | Electricity | Short
Derivative [Line Items]
Notional amount (in units) | kWh	502.5	502.5	366.7	366.7	265.0	265.0
Natural gas futures, forward and pipeline contracts (in dekatherms) | Natural Gas
Derivative [Line Items]
Notional amount (in units)	185.5	185.5	196.1	196.1	213.7	213.7
Natural gas basis swap contracts (in dekatherms) | Natural Gas
Derivative [Line Items]
Notional amount (in units)	155.4	155.4	131.1	131.1	84.0	84.0
NYMEX natural gas storage futures contracts (in dekatherms) | Natural Gas
Derivative [Line Items]
Notional amount (in units)	2.2	2.2	0.3	0.3	0.3	0.3
NYMEX natural gas option contracts (in dekatherms) | Natural Gas
Derivative [Line Items]
Notional amount (in units)	0.0	0.0	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	0.1	0.1
Interest rate swaps
Derivative [Line Items]
Notional amount	$ 1,350.7	€ 300.0	$ 1,357.3	€ 300.0	$ 114.1	€ 300.0
Forward foreign currency exchange contracts
Derivative [Line Items]
Notional amount	$ 460.1	€ 172.8	$ 516.0	€ 172.8	$ 299.1	€ 172.8
Regulated Utility Operations | Commodity contracts | Natural Gas
Derivative [Line Items]
Notional amount (in units)	11.3	11.3	23.3	23.3	11.6	11.6