Investment Securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Held-to-maturity, interest receivable	$ 1,000
Available-for-sale, interest receivable	17,100
Estimated Fair Value	3,473,598	$ 3,288,343
Securities valued at less than historical cost, amount	$ 559,281
Securities continuous unrealized loss position fair value (as percent)	16.10%
Decrease in available-for-sale provision for credit losses	$ (61)	0	$ 0
Securities pledged as collateral	2,000,000	1,700,000
Realized gains	54,800	13,300	65
Realized losses	$ 15	$ 4	$ 4
Income tax benefit related to security gains (losses) (as percent)	26.135%	26.135%	26.135%
Bonds
Debt Securities, Available-for-sale [Line Items]
Investments sold, amount	$ 1,700,000