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, 2016 EUR (€) gal MMBTU kWh	Dec. 31, 2016 USD ($) gal MMBTU kWh	Sep. 30, 2016 EUR (€) gal MMBTU kWh	Sep. 30, 2016 USD ($) gal MMBTU kWh	Dec. 31, 2015 EUR (€) gal MMBTU kWh	Dec. 31, 2015 USD ($) gal MMBTU kWh
Commodity contracts | Electricity | Long
Derivative
Notional amount (in units)	685.5	685.5	761.2	761.2	547.8	547.8
Commodity contracts | Electricity | Short
Derivative
Notional amount (in units)	352.5	352.5	264.6	264.6	252.9	252.9
Commodity contracts | Propane
Derivative
Notional amount (in units) | gal	325.9	325.9	396.9	396.9	481.9	481.9
Natural gas futures, forward and pipeline contracts | Natural Gas
Derivative
Notional amount (in units) | MMBTU	70.2	70.2	71.1	71.1	104.9	104.9
Natural gas basis swap contracts | Natural Gas
Derivative
Notional amount (in units) | MMBTU	120.1	120.1	118.3	118.3	86.1	86.1
NYMEX natural gas and propane storage | Natural Gas
Derivative
Notional amount (in units) | MMBTU	1.3	1.3	1.9	1.9	1.6	1.6
NYMEX natural gas and propane storage | Propane
Derivative
Notional amount (in units) | gal	0.0	0.0	0.0	0.0	1.8	1.8
FTRs | Electricity
Derivative
Notional amount (in units)	0.0	0.0	0.0	0.0	51.1	51.1
Interest rate swaps
Derivative
Notional amount | €	€ 645.8		€ 645.8		€ 645.8
IRPAs
Derivative
Notional amount | $		$ 0.0		$ 0.0		$ 290.0
Forward foreign currency exchange contracts
Derivative
Notional amount | $		416.7		314.3		280.5
Cross-currency swaps
Derivative
Notional amount | $		$ 59.1		$ 59.1		$ 59.1
Regulated Utility Operations | Commodity contracts | Natural Gas
Derivative
Notional amount (in units) | MMBTU	11.7	11.7	18.4	18.4	12.4	12.4
Regulated Utility Operations | Commodity contracts | Electricity
Derivative
Notional amount (in units)	0.0	0.0	0.0	0.0	55.9	55.9
Regulated Utility Operations | FTRs | Electricity
Derivative
Notional amount (in units)	36.2	36.2	58.3	58.3	172.6	172.6