Risk Management Activities (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015	Dec. 04, 2015	Dec. 03, 2015
Interest Rate Swap
Derivative [Line Items]
Notional Amount of Forward-Starting Interest rate Swap Agreements			$ 1,000.0	$ 500.0	$ 500.0
Swap Forecasted Debt Issuance			2018
Cash Flow Hedging
Derivative [Line Items]
Gain (Loss) on Cash Flow Hedge Ineffectiveness, Net	$ 0.0	$ 0.0
Derivative Liability, Fair Value, Gross Liability	132.5		$ 17.4
Price Risk Cash Flow Hedge Unrealized Gain (Loss) to be Reclassified During Next 12 Months	$ 0.0