Derivative Instruments and Hedging Activities - Schedule of Notional Amounts (Details) € in Millions, kWh in Millions, gal in Millions, MMBTU in Millions, $ in Millions	Dec. 31, 2017 EUR (€) gal MMBTU kWh	Dec. 31, 2017 USD ($) gal MMBTU kWh	Sep. 30, 2017 EUR (€) gal MMBTU kWh	Sep. 30, 2017 USD ($) gal MMBTU kWh	Dec. 31, 2016 EUR (€) gal MMBTU kWh	Dec. 31, 2016 USD ($) gal MMBTU kWh
Commodity contracts | Electricity | Long
Derivative [Line Items]
Notional amount (in units) | kWh	4,733.9	4,733.9	4,440.3	4,440.3	685.5	685.5
Commodity contracts | Electricity | Short
Derivative [Line Items]
Notional amount (in units) | kWh	325.2	325.2	447.0	447.0	352.5	352.5
Commodity contracts | Propane
Derivative [Line Items]
Notional amount (in units) | gal	275.4	275.4	325.5	325.5	325.9	325.9
Natural gas futures, forward and pipeline contracts (in dekatherms) | Natural Gas
Derivative [Line Items]
Notional amount (in units)	128.3	128.3	75.9	75.9	70.2	70.2
Natural gas basis swap contracts (in dekatherms) | Natural Gas
Derivative [Line Items]
Notional amount (in units)	90.2	90.2	104.2	104.2	120.1	120.1
NYMEX natural gas storage (in dekatherms) | Natural Gas
Derivative [Line Items]
Notional amount (in units)	1.3	1.3	1.9	1.9	1.3	1.3
NYMEX propane storage (in gallons) | Propane
Derivative [Line Items]
Notional amount (in units) | gal	0.1	0.1	0.3	0.3	0.0	0.0
Interest rate swaps
Derivative [Line Items]
Notional amount | €	€ 645.8		€ 645.8		€ 645.8
Forward foreign currency exchange contracts
Derivative [Line Items]
Notional amount | $		$ 485.7		$ 424.8		$ 416.7
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)	13.4	13.4	14.8	14.8	11.7	11.7
Regulated Utility Operations | FTRs (in kilowatts) | Electricity
Derivative [Line Items]
Notional amount (in units) | kWh	63.1	63.1	101.2	101.2	36.2	36.2