Securities Available for Sale and Held to Maturity - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Investments Debt And Equity Securities [Abstract]
Reclassification of Securities available for sale to securities held to maturity			$ 1,099,000,000
Net unrealized holding loss on AFS Securities at date of transfer			46,600,000
Net unrealized holding losses on AFS Securities, net of tax at date of transfer			28,800,000
Net unamortized, unrealized loss on transfer of securities	$ 34,000,000
Net unamortized, unrealized loss on transfer of securities, net of tax	21,000,000
Other-than-temporary impairments	0	$ 0	$ 0
Pledged to collateralize public deposits and securities sold under repurchase agreements and for other purposes as permitted by law	2,157,000,000	$ 2,457,000,000
Pledged securities providing additional contingency funding	$ 0