Derivative Instruments and Hedging Activities - Schedule of Notional Amounts (Details) € in Millions, kWh in Millions, gal in Millions, MMBTU in Millions, $ in Millions	Jun. 30, 2017 USD ($) gal kWh MMBTU	Jun. 30, 2017 EUR (€) gal kWh MMBTU	Sep. 30, 2016 USD ($) gal kWh MMBTU	Sep. 30, 2016 EUR (€) gal kWh MMBTU	Jun. 30, 2016 USD ($) gal kWh MMBTU	Jun. 30, 2016 EUR (€) gal kWh MMBTU
Commodity contracts | Electricity | Long
Derivative
Notional amount (in units) | kWh	686.3	686.3	761.2	761.2	558.0	558.0
Commodity contracts | Electricity | Short
Derivative
Notional amount (in units) | kWh	471.4	471.4	264.6	264.6	344.7	344.7
Commodity contracts | Propane
Derivative
Notional amount (in units) | gal	284.9	284.9	396.9	396.9	406.4	406.4
Natural gas futures, forward and pipeline contracts | Natural Gas
Derivative
Notional amount (in units)	55.5	55.5	71.1	71.1	79.6	79.6
Natural gas basis swap contracts | Natural Gas
Derivative
Notional amount (in units)	113.2	113.2	118.3	118.3	106.3	106.3
NYMEX natural gas storage | Natural Gas
Derivative
Notional amount (in units)	1.6	1.6	1.9	1.9	1.8	1.8
NYMEX propane storage | Propane
Derivative
Notional amount (in units) | gal	0.3	0.3	0.0	0.0	0.0	0.0
Interest rate swaps
Derivative
Notional amount | €		€ 645.8		€ 645.8		€ 645.8
Forward foreign currency exchange contracts
Derivative
Notional amount | $	$ 467.4		$ 314.3		$ 316.8
Cross-currency swaps
Derivative
Notional amount | $	$ 59.1		$ 59.1		$ 59.1
Regulated Utility Operations | Commodity contracts | Natural Gas
Derivative
Notional amount (in units)	12.7	12.7	18.4	18.4	13.4	13.4
Regulated Utility Operations | FTRs | Electricity
Derivative
Notional amount (in units) | kWh	139.4	139.4	58.3	58.3	80.6	80.6