Securities - Narrative (Details) $ in Millions	Sep. 30, 2016 USD ($)
States and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Portion of unrealized loss on state and political subdivisions securities that is the result of fair value hedge basis adjustments	$ 45
FNMA investments
Schedule of Available-for-sale Securities [Line Items]
Securities, amortized cost	14,900
Securities, fair value	14,900
FHLMC investments
Schedule of Available-for-sale Securities [Line Items]
Securities, amortized cost	8,300
Securities, fair value	$ 8,400