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, 2019 USD ($) DTH gal kWh	Jun. 30, 2019 EUR (€) DTH gal kWh	Sep. 30, 2018 USD ($) DTH gal kWh	Sep. 30, 2018 EUR (€) DTH gal kWh	Jun. 30, 2018 USD ($) DTH gal kWh	Jun. 30, 2018 EUR (€) DTH gal kWh
Commodity contracts | Electricity | Long
Derivative [Line Items]
Notional amount (in units) | kWh	3,140.0	3,140.0	4,307.6	4,307.6	4,283.0	4,283.0
Commodity contracts | Electricity | Short
Derivative [Line Items]
Notional amount (in units) | kWh	589.3	589.3	359.3	359.3	424.2	424.2
Commodity contracts | Propane
Derivative [Line Items]
Notional amount (in units) | gal	635.2	635.2	394.3	394.3	339.7	339.7
Natural gas futures, forward and pipeline contracts (in dekatherms) | Natural Gas
Derivative [Line Items]
Notional amount (in units)	191.4	191.4	159.7	159.7	139.6	139.6
Natural gas basis swap contracts (in dekatherms) | Natural Gas
Derivative [Line Items]
Notional amount (in units)	90.3	90.3	54.4	54.4	64.1	64.1
NYMEX natural gas storage (in dekatherms) | Natural Gas
Derivative [Line Items]
Notional amount (in units)	1.7	1.7	1.8	1.8	1.4	1.4
NYMEX propane storage (in gallons) | Propane
Derivative [Line Items]
Notional amount (in units) | gal	0.5	0.5	0.6	0.6	1.1	1.1
Interest rate swaps
Derivative [Line Items]
Notional amount	$ 114.1	€ 300.0	$ 114.1	€ 585.8	$ 0.0	€ 585.8
Forward foreign currency exchange contracts
Derivative [Line Items]
Notional amount	436.7	€ 172.8	512.2	€ 0.0	492.7	€ 0.0
Cross-currency contracts
Derivative [Line Items]
Notional amount | $	$ 0.0		$ 49.9		$ 49.9
Regulated Utility Operations | Commodity contracts | Natural Gas
Derivative [Line Items]
Notional amount (in units)	16.5	16.5	23.2	23.2	16.8	16.8