Investment Securities - Narrative (Details)	3 Months Ended			6 Months Ended
	Jun. 30, 2021 USD ($) investment_security	Mar. 31, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) investment_security	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) investment_security
Investment Securities
Investment securities pledged	$ 134,600,000			$ 134,600,000		$ 147,300,000
Accumulated other comprehensive income (loss) before tax amount	37,100,000			37,100,000		84,400,000
Accumulated other comprehensive income (loss), net of tax	26,524,000			26,524,000		60,292,000
Available-for-sale securities acquired				1,331,037,000	$ 458,667,000
Allowance for credit losses on debt securities, Held-to-maturity	0			0		0
Allowance for credit losses on debt securities, Available-for-sale	0			0		$ 0
Available-for-sale securities, provision for credit losses	$ 0	$ 0	$ 0	$ 0	0
Available-for-sale and held-to-maturity securities in nonaccrual status | investment_security	0			0		0
Available-for-sale and held-to-maturity securities purchased with deterioration in credit quality | investment_security	0			0		0
Available-for-sale and held-to-maturity collateral dependant | investment_security	0			0		0
Gross gains	$ 10,000,000.0	4,200,000	1,300,000	$ 14,300,000	9,200,000
Gross losses	5,000,000.0	191,000	1,300,000	5,100,000	1,500,000
Proceeds from available-for-sale securities	285,300,000	179,400,000	191,100,000	464,700,000	346,400,000
Receivable on unsettled security sales			6,500,000	0	$ 6,529,000
FHLB stock	17,300,000			17,300,000
FRB stock	74,400,000			74,400,000
Other stock	26,300,000			26,300,000
Amount of stock repurchased by FHLB	$ 0	$ 0	17,300,000
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