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, 2017 EUR (€) kWh gal MMBTU	Mar. 31, 2017 USD ($) kWh gal MMBTU	Sep. 30, 2016 EUR (€) kWh gal MMBTU	Sep. 30, 2016 USD ($) kWh gal MMBTU	Mar. 31, 2016 EUR (€) kWh gal MMBTU	Mar. 31, 2016 USD ($) kWh gal MMBTU
Commodity contracts | Electricity | Long
Derivative
Notional amount (in units)	668.7	668.7	761.2	761.2	591.1	591.1
Commodity contracts | Electricity | Short
Derivative
Notional amount (in units)	526.1	526.1	264.6	264.6	431.6	431.6
Commodity contracts | Propane
Derivative
Notional amount (in units) | gal	241.6	241.6	396.9	396.9	370.2	370.2
FTRs | Electricity
Derivative
Notional amount (in units)	0.0	0.0	0.0	0.0	20.5	20.5
Natural gas futures, forward and pipeline contracts | Natural Gas
Derivative
Notional amount (in units) | MMBTU	56.5	56.5	71.1	71.1	84.7	84.7
Natural gas basis swap contracts | Natural Gas
Derivative
Notional amount (in units) | MMBTU	107.2	107.2	118.3	118.3	90.1	90.1
NYMEX natural gas and propane storage | Natural Gas
Derivative
Notional amount (in units) | MMBTU	1.4	1.4	1.9	1.9	1.7	1.7
Interest rate swaps
Derivative
Notional amount | €	€ 645.8		€ 645.8		€ 645.8
Forward foreign currency exchange contracts
Derivative
Notional amount | $		$ 321.8		$ 314.3		$ 262.5
Cross-currency swaps
Derivative
Notional amount | $		$ 59.1		$ 59.1		$ 59.1
Regulated Utility Operations | Commodity contracts | Natural Gas
Derivative
Notional amount (in units) | MMBTU	9.0	9.0	18.4	18.4	10.0	10.0
Regulated Utility Operations | Commodity contracts | Electricity
Derivative
Notional amount (in units)	0.0	0.0	0.0	0.0	23.6	23.6
Regulated Utility Operations | FTRs | Electricity
Derivative
Notional amount (in units)	14.6	14.6	58.3	58.3	69.2	69.2