Investment Securities - Textual (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Securities valued at less than historical cost, amount	$ 1,400,000,000		$ 1,700,000,000
Securities valued at less than historical cost (as percent)	60.10%		70.30%
Securities pledged as collateral	$ 1,060,000,000		$ 1,020,000,000
Realized gains	2,700,000	$ 6,000
Realized losses	$ 0	$ 0