Securities Available for Sale and Held to Maturity - Additional Information (Details) - USD ($)	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	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	$ 16,600,000
Net unamortized, unrealized loss on transfer of securities, net of tax	12,400,000
Other-than-temporary impairments	0	$ 0
Pledged to collateralize public deposits and securities sold under repurchase agreements and for other purposes as permitted by law	2,092,000,000		$ 1,834,000,000
Pledged securities providing additional contingency funding	$ 0		$ 0