Derivative Instruments and Hedging Activities - Schedule of Notional Amounts (Details) € in Millions, kWh in Millions, gal in Millions, MMBTU in Millions, $ in Millions	Sep. 30, 2017 EUR (€) kWh gal MMBTU	Sep. 30, 2017 USD ($) kWh gal MMBTU	Sep. 30, 2016 EUR (€) kWh gal MMBTU	Sep. 30, 2016 USD ($) kWh gal MMBTU
Commodity contracts | Electricity | Long
Derivative
Notional amount (in units) | kWh	4,440.3	4,440.3	761.2	761.2
Commodity contracts | Electricity | Short
Derivative
Notional amount (in units) | kWh	447.0	447.0	264.6	264.6
Commodity contracts | Propane
Derivative
Notional amount (in units) | gal	325.5	325.5	396.9	396.9
Natural gas futures, forward and pipeline contracts | Natural Gas
Derivative
Notional amount (in units)	75.9	75.9	71.1	71.1
Natural gas basis swap contracts | Natural Gas
Derivative
Notional amount (in units)	104.2	104.2	118.3	118.3
NYMEX natural gas storage | Natural Gas
Derivative
Notional amount (in units)	1.9	1.9	1.9	1.9
NYMEX propane storage | Propane
Derivative
Notional amount (in units) | gal	0.3	0.3	0.0	0.0
Interest rate swaps
Derivative
Notional amount | €	€ 645.8		€ 645.8
Forward foreign currency exchange contracts
Derivative
Notional amount | $		$ 424.8		$ 314.3
Cross-currency swaps
Derivative
Notional amount | $		$ 59.1		$ 59.1
Regulated Utility Operations | Commodity contracts | Natural Gas
Derivative
Notional amount (in units)	14.8	14.8	18.4	18.4
Regulated Utility Operations | FTRs contracts | Electricity
Derivative
Notional amount (in units) | kWh	101.2	101.2	58.3	58.3