Investment Securities - Textual (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Securities valued at less than historical cost, amount	$ 1,800,000		$ 1,800,000		$ 1,400,000
Securities valued at less than historical cost (as percent)	80.20%		80.20%		73.50%
Securities pledged as collateral	$ 1,500,000		$ 1,500,000		$ 1,200,000
Realized gains	7	$ 2,200	13	$ 2,300
Realized losses	$ (14)	$ 5	$ (14)	$ 5