Securities Securities - Narrative (Details) - USD ($)	3 Months Ended			9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2020	Sep. 30, 2019	Jan. 01, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities transferred to held-to-maturity portfolio			$ 179,800,000	$ 873,613,000	$ 0
Unrealized after tax gain on available for sale securities transferred to the held to maturity portfolio	$ 69,000,000.0			69,000,000.0
Securities pledged as collateral to secure public funds and other deposits or securities sold under agreements to repurchase	1,200,000,000			1,200,000,000
Secured borrowing under agreement to repurchase	127,100,000			127,100,000
Accumulated unrealized losses	35,288,000			35,288,000			$ 17,785,000
Fair value of securities	1,151,426,000			1,151,426,000			1,297,321,000
Allowance for credit losses - securities held-to-maturity	(191,000)			(191,000)		$ (10,000)	$ 0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	481,000	$ 308,000		728,000	(4,438,000)
Proceeds from Sale of Available-for-sale Securities				145,631,000	$ 626,097,000
Securities pledged as collateral
Debt Securities, Available-for-sale [Line Items]
Secured borrowing under agreement to repurchase	$ 127,100,000			$ 127,100,000