Investment Securities - Narrative (Details)	3 Months Ended				9 Months Ended
	Sep. 30, 2021 USD ($) investment_security	Jun. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) investment_security	Sep. 30, 2020 USD ($)	Jul. 01, 2021 USD ($)	Dec. 31, 2020 USD ($) investment_security
Investment Securities
Net carrying amount of securities transferred from available-for-sale to held-to-maturity					$ 154,456,000	$ 0
Unrealized loss on securities transferred from available-for-sale to held-to-maturity	$ 2,225,000				2,225,000			$ 0
Investment securities pledged	131,800,000				131,800,000			147,300,000
Accumulated other comprehensive income (loss), net of tax	1,176,000				1,176,000			60,292,000
Available-for-sale securities acquired					2,079,198,000	2,057,690,000
Net Carrying Amount	11,000				11,000			0
Held-to-maturity securities, provision for credit losses	11,000	$ 0	$ 0		11,000	0
Available-for-sale securities, provision for credit losses	0	0	0		0	0
Allowance for credit losses on debt securities, Available-for-sale	$ 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 dependent | investment_security	0				0			0
Gross gains	$ 4,200,000	10,000,000	1,200,000		$ 18,500,000	10,400,000
Gross losses	0	5,000,000	12,000		5,100,000	1,500,000
Proceeds from available-for-sale securities	165,800,000	285,300,000	212,500,000		630,400,000	$ 558,900,000
FHLB stock	17,300,000				17,300,000
FRB stock	74,500,000				74,500,000
Other stock	26,700,000				26,700,000
Amount of stock repurchased by FHLB	0	$ 0	$ 0
Impairment loss on investments in FHLB, FRB and other stock					0
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent
Investment Securities
Accumulated other comprehensive income (loss) before tax amount	4,700,000				4,700,000			$ 84,400,000
Accumulated other comprehensive income (loss), net of tax	3,400,000				3,400,000			60,300,000
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
Mortgage-backed securities
Investment Securities
Unrealized loss on securities transferred from available-for-sale to held-to-maturity	0				0			$ 0
Municipal bonds
Investment Securities
Securities transferred from available-for-sale to held-to-maturity	157,600,000
Net carrying amount of securities transferred from available-for-sale to held-to-maturity	154,500,000
Unrealized loss on securities transferred from available-for-sale to held-to-maturity	2,225,000				2,225,000		$ 3,200,000
Accumulated other comprehensive income (loss) before tax amount	(3,100,000)				(3,100,000)
Accumulated other comprehensive income (loss), net of tax	$ (2,200,000)				$ (2,200,000)