Investment Securities - Narrative (Details)	3 Months Ended			6 Months Ended
	Jun. 30, 2020 USD ($) security	Mar. 31, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) security	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) security
Investment Securities
Accumulated other comprehensive income (loss) before tax amount	$ 81,600,000			$ 81,600,000		$ 30,100,000
Accumulated other comprehensive income (loss), net of tax	$ 58,224,000			58,224,000		$ 21,523,000
Available-for-sale securities, provision for credit losses				$ 0
Other than temporary impairment losses recognized in earnings					$ 0
Available-for-sale and held-to-maturity securities in nonaccrual status | security	0			0
Available-for-sale and held-to-maturity securities purchased with deterioration in credit quality | security	0			0		0
Available-for-sale and held-to-maturity collateral dependant | security	0			0
Gross gains	$ 1,300,000	$ 8,000,000.0	$ 406,000	$ 9,200,000	1,400,000
Gross losses	1,300,000	204,000	194,000	1,500,000	809,000
Proceeds from available-for-sale securities	191,100,000	155,300,000	57,200,000	346,400,000	227,100,000
Receivables for securities with a later settlement date	6,500,000			6,500,000
Receivable on unsettled security sales				6,529,000	$ 0
Investment securities pledged	133,300,000			133,300,000		$ 125,700,000
FHLB stock	17,300,000			17,300,000
FRB stock	51,800,000			51,800,000
Other stock	25,700,000			25,700,000
Amount of stock repurchased by FHLB	$ 17,300,000	$ 10,300,000	$ 5,400,000
Impairment loss on investments in FHLB, FRB and other stock				$ 0