Investment Securities - Textual (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Held-to-maturity, interest receivable	$ 247,000		$ 247,000
Available-for-sale, interest receivable	12,700,000		12,700,000
Available-for-sale securities	2,496,896,000		2,496,896,000		$ 3,288,343,000
Securities valued at less than historical cost, amount	$ 141,718,000		$ 141,718,000
Securities valued at less than historical cost (as percent)	5.70%		5.70%
Decrease in available-for-sale provision for credit losses	$ 340,000		$ 236,000
Securities pledged as collateral	1,380,000,000		1,380,000,000		$ 1,730,000,000
Realized gains	391,000	$ 2,800,000	32,500,000	$ 5,600,000
Realized losses	$ (1,000)	$ 0	(2,600)	$ 0
Investments sold, amount			$ 1,200,000,000
Income tax benefit related to security gains (losses) (as percent)			26.135%	26.135%