Derivative Instruments and Hedging Activities - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Derivative, Collateral, Right to Reclaim Cash	$ 304	$ 87
Additional collateral, independent margin	55	65
Derivative net liability position (less than $1 million)	1	1
Cash collateral from derivatives counterparties	544	695
Securities received from derivatives counterparties, ability to re-pledge	172	91
Cumulative counterparty credit risk valuation adjustment	4	5
Cumulative credit risk valuation adjustment related to our credit quality (less than $1 million for 2015)	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	$ 109
Maximum length of time over which forecasted transactions were hedged, years	6 years