Derivative Instruments and Hedging Activities - Schedule of Notional Amounts (Details) € in Millions, kWh in Millions, gal in Millions, DTH in Millions, $ in Millions	Jun. 30, 2020 USD ($) gal DTH kWh	Jun. 30, 2020 EUR (€) gal DTH kWh	Sep. 30, 2019 USD ($) DTH kWh gal	Sep. 30, 2019 EUR (€) DTH kWh gal	Jun. 30, 2019 USD ($) gal DTH kWh	Jun. 30, 2019 EUR (€) gal DTH kWh
Commodity contracts | Propane
Derivative [Line Items]
Notional amount (in units) | gal	889.6	889.6	800.4	800.4	635.2	635.2
Commodity contracts | Electricity | Long
Derivative [Line Items]
Notional amount (in units) | kWh	4,549.5	4,549.5	3,098.1	3,098.1	3,140.0	3,140.0
Commodity contracts | Electricity | Short
Derivative [Line Items]
Notional amount (in units) | kWh	405.5	405.5	366.7	366.7	589.3	589.3
Natural gas futures, forward and pipeline contracts (in dekatherms) | Natural Gas
Derivative [Line Items]
Notional amount (in units)	180.8	180.8	196.1	196.1	191.4	191.4
Natural gas basis swap contracts (in dekatherms) | Natural Gas
Derivative [Line Items]
Notional amount (in units)	159.0	159.0	131.1	131.1	90.3	90.3
NYMEX natural gas storage (in dekatherms) | Natural Gas
Derivative [Line Items]
Notional amount (in units)	2.5	2.5	0.3	0.3	1.7	1.7
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 (in gallons) | Propane
Derivative [Line Items]
Notional amount (in units) | gal	0.9	0.9	0.5	0.5	0.5	0.5
Interest rate swaps
Derivative [Line Items]
Notional amount	$ 1,347.4	€ 300.0	$ 1,357.3	€ 300.0	$ 114.1	€ 300.0
Forward foreign currency exchange contracts
Derivative [Line Items]
Notional amount	$ 439.5	€ 172.8	$ 516.0	€ 172.8	$ 436.7	€ 172.8
Regulated Utility Operations | Commodity contracts | Natural Gas
Derivative [Line Items]
Notional amount (in units)	15.5	15.5	23.3	23.3	16.5	16.5