Investment Securities - Textual (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Held-to-maturity, interest receivable	$ 334,000
Available-for-sale, interest receivable	9,700,000
Available-for-sale securities	2,466,640,000		$ 3,288,343,000
Securities valued at less than historical cost, amount	$ 640,524,000
Securities valued at less than historical cost (as percent)	26.00%
Decrease in available-for-sale provision for credit losses	$ (104,000,000)
Securities pledged as collateral	1,740,000,000	$ 2,700,000	$ 1,730,000,000
Realized gains	32,100,000
Realized losses	(2,080)	$ 0
Investments sold, amount	$ 1,100,000,000
Income tax benefit related to security gains (losses) (as percent)			26.135%