Investment Securities - Textual (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Securities valued at less than historical cost, amount	$ 2,000,000,000		$ 2,000,000,000		$ 1,400,000,000
Securities valued at less than historical cost (as percent)	86.00%		86.00%		73.50%
Securities pledged as collateral	$ 933,500,000		$ 933,500,000		$ 1,200,000,000
Realized gains	41,000	$ 3,000	54,000	$ 2,300,000
Realized losses	$ 0	$ 0	$ 1,000	$ 5,000