Fair Value Accounting - Changes in the Fair Value of Level 3 Financial Assets (Details) $ in Millions	9 Months Ended
	Sep. 30, 2015 USD ($)
Summary of changes in Level 3 financial assets
Balance at beginning of period, assets	$ 30
Revaluation	(4)
Balance at end of period, assets	26
Auction rate securities
Summary of changes in Level 3 financial assets
Balance at beginning of period, assets	6
Revaluation	1
Balance at end of period, assets	7
Asset backed commercial paper
Summary of changes in Level 3 financial assets
Balance at beginning of period, assets	24
Revaluation	(5)
Balance at end of period, assets	$ 19