Derivative Instruments and Hedging Activities - Schedule of Notional Amounts (Details) € in Millions, kWh in Millions, gal in Millions, DTH in Millions, $ in Millions	Sep. 30, 2019 USD ($) DTH kWh gal	Sep. 30, 2019 EUR (€) DTH kWh gal	Sep. 30, 2018 USD ($) DTH kWh gal	Sep. 30, 2018 EUR (€) DTH kWh gal
Commodity contracts | Propane
Derivative
Notional amount (in units) | gal	800.4	800.4	394.3	394.3
Commodity contracts | Electricity | Long
Derivative
Notional amount (in units) | kWh	3,098.1	3,098.1	4,307.6	4,307.6
Commodity contracts | Electricity | Short
Derivative
Notional amount (in units) | kWh	366.7	366.7	359.3	359.3
Natural gas futures, forward and pipeline contracts | Natural Gas
Derivative
Notional amount (in units)	196.1	196.1	159.7	159.7
Natural gas basis swap contracts | Natural Gas
Derivative
Notional amount (in units)	131.1	131.1	54.4	54.4
NYMEX natural gas storage futures contracts | Natural Gas
Derivative
Notional amount (in units)	0.3	0.3	1.8	1.8
NYMEX natural gas option contracts | Natural Gas
Derivative
Notional amount (in units)	2.4	2.4	0.0	0.0
NYMEX propane storage futures contracts | Propane
Derivative
Notional amount (in units) | gal	0.5	0.5	0.6	0.6
Interest rate swaps
Derivative
Notional amount	$ 1,357.3	€ 300.0	$ 114.1	€ 585.8
Forward foreign exchange contracts
Derivative
Notional amount | $	516.0		512.2
Cross-currency swaps
Derivative
Notional amount | $	$ 0.0		$ 49.9
Regulated Utility Operations | Commodity contracts | Natural Gas
Derivative
Notional amount (in units)	23.3	23.3	23.2	23.2
Net Investment Hedging | Forward foreign exchange contracts
Derivative
Notional amount	$ 0.0	€ 172.8		€ 0.0