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, 2019 USD ($) DTH gal kWh	Mar. 31, 2019 EUR (€) DTH gal kWh	Sep. 30, 2018 USD ($) DTH gal kWh	Sep. 30, 2018 EUR (€) DTH gal kWh	Mar. 31, 2018 USD ($) DTH gal kWh	Mar. 31, 2018 EUR (€) DTH gal kWh
Commodity contracts | Electricity | Long
Derivative [Line Items]
Notional amount (in units) | kWh	3,685.0	3,685.0	4,307.6	4,307.6	4,184.1	4,184.1
Commodity contracts | Electricity | Short
Derivative [Line Items]
Notional amount (in units) | kWh	265.0	265.0	359.3	359.3	490.9	490.9
Commodity contracts | Propane
Derivative [Line Items]
Notional amount (in units) | gal	369.2	369.2	394.3	394.3	229.2	229.2
Natural gas futures, forward and pipeline contracts (in dekatherms) | Natural Gas
Derivative [Line Items]
Notional amount (in units)	213.7	213.7	159.7	159.7	128.9	128.9
Natural gas basis swap contracts (in dekatherms) | Natural Gas
Derivative [Line Items]
Notional amount (in units)	84.0	84.0	54.4	54.4	74.1	74.1
NYMEX natural gas storage (in dekatherms) | Natural Gas
Derivative [Line Items]
Notional amount (in units)	0.3	0.3	1.8	1.8	0.9	0.9
NYMEX propane storage (in gallons) | Propane
Derivative [Line Items]
Notional amount (in units) | gal	0.1	0.1	0.6	0.6	0.0	0.0
Interest rate swaps
Derivative [Line Items]
Notional amount	$ 114.1	€ 300.0	$ 114.1	€ 585.8	$ 0.0	€ 645.8
Forward foreign currency exchange contracts
Derivative [Line Items]
Notional amount	299.1	€ 172.8	512.2	€ 0.0	496.2	€ 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)	11.6	11.6	23.2	23.2	12.7	12.7
Regulated Utility Operations | FTRs contracts (Kilowatt hours) | Electricity
Derivative [Line Items]
Notional amount (in units) | kWh	0.0	0.0	0.0	0.0	25.5	25.5