Derivative Instruments (Details) $ in Millions	6 Months Ended		12 Months Ended
	Jun. 30, 2016 USD ($) MMBTU MWh t		Dec. 31, 2014 MMBTU MWh t	Dec. 31, 2015 USD ($)
Power [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh	(48)		(41)
Natural Gas [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	1,038		996
Environmental Credits [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Mass | t	18		8
Interest Rate Hedging Instruments
Derivative [Line Items]
Derivative, Notional Amount | $	$ 3,798	[1]		$ 1,320

[1] We entered into interest rate hedging instruments during the second quarter of 2016 to hedge approximately $2.5 billion of variable rate corporate debt for 2017 through 2019 which effectively places a ceiling on LIBOR at rates varying from 1.44% to 1.8125% for hedged interest payments. See Note 4 for a further discussion of our First Lien Term Loans.