Derivative Instruments and Hedging Activities - Schedule of Notional Amounts (Details) € in Millions, kWh in Millions, gal in Millions, MMBTU in Millions, $ in Millions	Mar. 31, 2018 EUR (€) gal kWh MMBTU	Mar. 31, 2018 USD ($) gal kWh MMBTU	Sep. 30, 2017 EUR (€) gal kWh MMBTU	Sep. 30, 2017 USD ($) gal kWh MMBTU	Mar. 31, 2017 EUR (€) gal kWh MMBTU	Mar. 31, 2017 USD ($) gal kWh MMBTU
Commodity contracts | Electricity | Long
Derivative [Line Items]
Notional amount (in units) | kWh	4,184.1	4,184.1	4,440.3	4,440.3	668.7	668.7
Commodity contracts | Electricity | Short
Derivative [Line Items]
Notional amount (in units) | kWh	490.9	490.9	447.0	447.0	526.1	526.1
Commodity contracts | Propane
Derivative [Line Items]
Notional amount (in units) | gal	229.2	229.2	325.5	325.5	241.6	241.6
Natural gas futures, forward and pipeline contracts (in dekatherms) | Natural Gas
Derivative [Line Items]
Notional amount (in units)	128.9	128.9	75.9	75.9	56.5	56.5
Natural gas basis swap contracts (in dekatherms) | Natural Gas
Derivative [Line Items]
Notional amount (in units)	74.1	74.1	104.2	104.2	107.2	107.2
NYMEX natural gas storage (in dekatherms) | Natural Gas
Derivative [Line Items]
Notional amount (in units)	0.9	0.9	1.9	1.9	1.4	1.4
NYMEX propane storage (in gallons) | Propane
Derivative [Line Items]
Notional amount (in units) | gal	0.0	0.0	0.3	0.3	0.0	0.0
Interest rate swaps
Derivative [Line Items]
Notional amount | €	€ 645.8		€ 645.8		€ 645.8
Forward foreign currency exchange contracts
Derivative [Line Items]
Notional amount | $		$ 496.2		$ 424.8		$ 321.8
Cross-currency contracts
Derivative [Line Items]
Notional amount | $		$ 49.9		$ 59.1		$ 59.1
Regulated Utility Operations | Commodity contracts | Natural Gas
Derivative [Line Items]
Notional amount (in units)	12.7	12.7	14.8	14.8	9.0	9.0
Regulated Utility Operations | FTRs (in kilowatts) | Electricity
Derivative [Line Items]
Notional amount (in units) | kWh	25.5	25.5	101.2	101.2	14.6	14.6