Investment Securities - Narrative (Details)	3 Months Ended			9 Months Ended
	Sep. 30, 2020 USD ($) investmentSecurity	Jun. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) investmentSecurity	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) investmentSecurity
Investment Securities
Accumulated other comprehensive income (loss) before tax amount	$ 64,000,000.0			$ 64,000,000.0		$ 30,100,000
Accumulated other comprehensive income (loss), net of tax	$ 45,663,000			45,663,000		$ 21,523,000
Available-for-sale securities acquired				2,057,690,000	$ 603,457,000
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 | investmentSecurity	0			0
Available-for-sale and held-to-maturity securities purchased with deterioration in credit quality | investmentSecurity	0			0		0
Available-for-sale and held-to-maturity collateral dependant | investmentSecurity	0			0
Gross gains	$ 1,200,000	$ 1,300,000	$ 5,100,000	$ 10,400,000	6,500,000
Gross losses	12,000	1,300,000	811,000	1,500,000	1,600,000
Proceeds from available-for-sale securities	212,500,000	191,100,000	191,300,000	558,900,000	$ 418,500,000
Receivables for securities with a later settlement date		6,500,000
Investment securities pledged	149,000,000.0			149,000,000.0		$ 125,700,000
FHLB stock	17,300,000			17,300,000
FRB stock	51,700,000			51,700,000
Other stock	25,700,000			25,700,000
Amount of stock repurchased by FHLB	$ 0	17,300,000	$ 5,400,000
Impairment loss on investments in FHLB, FRB and other stock				$ 0
Opus Bank
Investment Securities
Available-for-sale securities acquired		829,900,000
Allowance for credit losses of available-for-sale securities classified as purchase credit deteriorated		0
Opus Bank | Financial Asset Acquired with Credit Deterioration
Investment Securities
Available-for-sale securities acquired		$ 0