Securities Securities - Narrative (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Sep. 30, 2018	Jan. 01, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities transferred to held-to-maturity portfolio	$ 873,613,000	$ 0	$ 179,800,000
Unrealized after tax gain on available for sale securities transferred to the held to maturity portfolio	69,000,000.0
Securities pledged as collateral to secure public funds and other deposits or securities sold under agreements to repurchase	1,300,000,000
Secured borrowing under agreement to repurchase	186,500,000
Accumulated unrealized losses	62,909,000				$ 17,785,000
Fair value of securities	997,981,000				$ 1,297,321,000
Allowance for credit losses - securities held-to-maturity	(148,000)			$ (10,000)
Debt Securities Available-for-sale Sold Amount	30,200,000
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	342,000	$ (1,448,000)
Securities pledged as collateral
Debt Securities, Available-for-sale [Line Items]
Secured borrowing under agreement to repurchase	$ 186,500,000