Derivative Instruments and Hedging Activities - Additional Information (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Derivative, Collateral, Right to Reclaim Cash	$ 157	$ 87
Additional collateral, independent margin	56	65
Derivative net liability position (less than $1 million)	1	1
Cash collateral from derivatives counterparties	838	695
Securities received from derivatives counterparties, ability to re-pledge	231	91
Cumulative counterparty credit risk valuation adjustment	4	5
Cumulative credit risk valuation adjustment related to our credit quality	1	$ 1
Gain (net after-tax) recorded in AOCI related to derivatives designated as cash flow hedges expected to be reclassified to earnings over the next 12 months	$ 129
Maximum length of time over which forecasted transactions were hedged, years	5 years