Derivatives and Risk Management Activities (Details 2) - Income Statement Location [Domain] $ in Millions	1 Months Ended	6 Months Ended
	Jun. 30, 2015 USD ($) contract	Jun. 30, 2015 USD ($) contract	Jun. 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	$ (673)	$ (673)	$ (142)	$ (467)	$ (97)
Cash paid in connection with termination of interest rate derivatives		29	$ 7
Interest Rate Derivatives
Interest Rate Risk Hedging
Net deferred gains (losses) from interest rate risk hedging included in AOCI	$ (109)	$ (109)
7 forward starting interest rate swaps (30-year) | Cash flow hedge
Interest Rate Risk Hedging
Number of interest rate derivatives (in contracts) | contract	7	7
Notional amount of derivatives	$ 250	$ 250
Rate of fixed interest to be received on interest rate swap (as a percent)	3.02%	3.02%
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	8
Notional amount of derivatives	$ 200	$ 200
Rate of fixed interest to be received on interest rate swap (as a percent)	3.06%	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	8
Notional amount of derivatives	$ 200	$ 200
Rate of fixed interest to be received on interest rate swap (as a percent)	3.14%	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	8
Notional amount of derivatives	$ 200	$ 200
Rate of fixed interest to be received on interest rate swap (as a percent)	3.20%	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	8
Notional amount of derivatives	$ 200	$ 200
Rate of fixed interest to be received on interest rate swap (as a percent)	2.83%	2.83%
10 forward starting interest rate swaps | Cash flow hedge
Interest Rate Risk Hedging
Number of interest rate derivatives terminated (in contracts) | contract	10
Notional amount of derivatives	$ 250	$ 250
Rate of fixed interest to be received on interest rate swap (as a percent)	3.60%	3.60%
Cash paid in connection with termination of interest rate derivatives	$ 31