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, 2018 EUR (€) gal kWh MMBTU	Jun. 30, 2018 USD ($) gal kWh MMBTU	Sep. 30, 2017 EUR (€) gal kWh MMBTU	Sep. 30, 2017 USD ($) gal kWh MMBTU	Jun. 30, 2017 EUR (€) gal kWh MMBTU	Jun. 30, 2017 USD ($) gal kWh MMBTU
Commodity contracts | Electricity | Long
Derivative [Line Items]
Notional amount (in units) | kWh	4,283.0	4,283.0	4,440.3	4,440.3	686.3	686.3
Commodity contracts | Electricity | Short
Derivative [Line Items]
Notional amount (in units) | kWh	424.2	424.2	447.0	447.0	471.4	471.4
Commodity contracts | Propane
Derivative [Line Items]
Notional amount (in units) | gal	339.7	339.7	325.5	325.5	284.9	284.9
Natural gas futures, forward and pipeline contracts (in dekatherms) | Natural Gas
Derivative [Line Items]
Notional amount (in units)	139.6	139.6	75.9	75.9	55.5	55.5
Natural gas basis swap contracts (in dekatherms) | Natural Gas
Derivative [Line Items]
Notional amount (in units)	64.1	64.1	104.2	104.2	113.2	113.2
NYMEX natural gas storage (in dekatherms) | Natural Gas
Derivative [Line Items]
Notional amount (in units)	1.4	1.4	1.9	1.9	1.6	1.6
NYMEX propane storage (in gallons) | Propane
Derivative [Line Items]
Notional amount (in units) | gal	1.1	1.1	0.3	0.3	0.3	0.3
Interest rate swaps
Derivative [Line Items]
Notional amount | €	€ 585.8		€ 645.8		€ 645.8
Forward foreign currency exchange contracts
Derivative [Line Items]
Notional amount | $		$ 492.7		$ 424.8		$ 467.4
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)	16.8	16.8	14.8	14.8	12.7	12.7
Regulated Utility Operations | FTRs (in kilowatts) | Electricity
Derivative [Line Items]
Notional amount (in units) | kWh	0.0	0.0	101.2	101.2	139.4	139.4