Derivatives and Risk Management Activities (Details 2) $ in Millions	1 Months Ended		3 Months Ended	9 Months Ended
	Aug. 31, 2015 USD ($) contract	Jun. 30, 2015 USD ($) contract	Aug. 31, 2015 USD ($)	Sep. 30, 2015 USD ($) contract	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Interest Rate Risk Hedging
Net deferred gains/(losses) from interest rate risk hedging included in AOCI				$ (985)	$ (308)	$ (467)	$ (97)
Cash paid in connection with termination of interest rate derivatives				48	$ 7
Interest Rate Derivatives
Interest Rate Risk Hedging
Net deferred gains/(losses) from interest rate risk hedging included in AOCI				$ (192)
8 forward starting interest rate swaps (30-year), one | Cash flow hedge
Interest Rate Risk Hedging
Number of interest rate derivatives (in contracts) | contract				8
Notional amount of derivatives				$ 200
Rate of fixed interest to be received on interest rate swap (as a percent)				3.06%
8 forward starting interest rate swaps (30-year), two | Cash flow hedge
Interest Rate Risk Hedging
Number of interest rate derivatives (in contracts) | contract				8
Notional amount of derivatives				$ 200
Rate of fixed interest to be received on interest rate swap (as a percent)				3.14%
8 forward starting interest rate swaps (30-year), three | Cash flow hedge
Interest Rate Risk Hedging
Number of interest rate derivatives (in contracts) | contract				8
Notional amount of derivatives				$ 200
Rate of fixed interest to be received on interest rate swap (as a percent)				3.20%
8 forward starting interest rate swaps (30-year), four | Cash flow hedge
Interest Rate Risk Hedging
Number of interest rate derivatives (in contracts) | contract				8
Notional amount of derivatives				$ 200
Rate of fixed interest to be received on interest rate swap (as a percent)				2.83%
10 forward starting interest rate swaps
Interest Rate Risk Hedging
Number of interest rate derivatives terminated (in contracts) | contract		10
Notional amount of derivatives		$ 250
Rate of fixed interest to be received on interest rate swap (as a percent)		3.60%
Cash paid in connection with termination of interest rate derivatives		$ 31
7 forward starting interest rate swaps
Interest Rate Risk Hedging
Number of interest rate derivatives terminated (in contracts) | contract	7
Notional amount of derivatives	$ 250		$ 250
Rate of fixed interest to be received on interest rate swap (as a percent)	3.03%		3.03%
Cash paid in connection with termination of interest rate derivatives	$ 21
Hedged Transactions No Longer Probable of Occurring | Interest expense
Interest Rate Risk Hedging
Gain/(loss) reclassified from AOCI into income			$ (4)