Securities Available for Sale and Held to Maturity - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	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	$ 25,700,000		$ 25,700,000
Net unamortized, unrealized loss on transfer of securities, net of tax	15,800,000		15,800,000
Other-than-temporary impairments	0	$ 0	0	$ 0
Pledged to collateralize public deposits and securities sold under repurchase agreements and for other purposes as permitted by law	1,826,000,000		1,826,000,000		$ 2,157,000,000
Pledged securities providing additional contingency funding	$ 0		$ 0		$ 0