Securities (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2020
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities transferred to Held-to-Maturity	$ 873,600,000	$ 179,800,000	$ 873,613,000	$ 0	$ 179,763,000
Unrealized after tax gain (loss) on available-for-sale securities transferred to the held-to-maturity portfolio	$ 69,000,000.0	$ (2,200,000)
Allowance for credit losses - securities held-to-maturity			(191,000)			$ (10,000)
Securities pledged as collateral to secure public funds and other deposits or securities sold under agreements to repurchase			1,200,000,000
Securities Loaned and Securities Sold under Agreement to Repurchase, Gross Including Not Subject to Master Netting Arrangement			128,200,000
Proceeds from Sale of Available-for-sale Securities			145,631,000	737,717,000	169,850,000
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax			(728,000)	$ (4,388,000)	$ (1,665,000)
Securities pledged as collateral
Debt Securities, Available-for-sale [Line Items]
Securities Loaned and Securities Sold under Agreement to Repurchase, Gross Including Not Subject to Master Netting Arrangement			$ 128,200,000