Investment Securities - Textual (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Held-to-maturity, interest receivable	$ 291,000		$ 291,000
Available-for-sale, interest receivable	12,100,000		12,100,000
Available-for-sale securities	2,607,288,000		2,607,288,000		$ 3,288,343,000
Securities valued at less than historical cost, amount	$ 475,133,000		$ 475,133,000
Securities valued at less than historical cost (as percent)	18.20%		18.20%
Decrease in available-for-sale provision for credit losses	$ 599,000		$ 835,000
Securities pledged as collateral	1,410,000,000		1,410,000,000		$ 1,730,000,000
Realized gains	22,300,000	$ 7,600,000	54,800,000	$ 12,900,000
Realized losses	$ 1,700	$ 3,000	4,400	$ 3,000
Investments sold, amount			$ 1,700,000,000
Income tax benefit related to security gains (losses) (as percent)			26.135%	26.135%