Acquisitions - Narrative (Details) $ / shares in Units, client_account in Thousands, $ in Thousands			3 Months Ended			6 Months Ended	9 Months Ended		15 Months Ended
	Jun. 01, 2020 USD ($) office $ / shares shares	May 29, 2020 USD ($) $ / shares	Sep. 30, 2021 USD ($)	Jun. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2021 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)	May 31, 2020 USD ($) client_account	Dec. 31, 2019 USD ($)
Business Acquisition [Line Items]
Total assets			$ 21,005,211				$ 21,005,211		$ 21,005,211	$ 19,736,544
Total gross loans			13,982,861				13,982,861		13,982,861	13,236,433
Total deposits			17,469,999				17,469,999		17,469,999	16,214,177
Goodwill			901,312		$ 898,434		901,312	$ 898,434	901,312	$ 898,569		$ 808,322
Purchase accounting adjustments to goodwill							2,743	(2,732)
ACL expense (reversal)			(19,543)		4,702		(52,359)	180,341
Initial ACL Recorded for PCD Loans								21,242
Merger-related expense			$ 0	$ 0	2,988		5	44,058
PENSCO
Business Acquisition [Line Items]
Custodial assets											$ 14,480,000
Number of client accounts | client_account											44
Opus Bank
Business Acquisition [Line Items]
Equity issued, ratio	0.900
Stock transaction value (in dollars per share) | $ / shares	$ 19.31
Closing price of Corporation's common stock (in dollars per share) | $ / shares		$ 21.62
Business combination, consideration transferred	$ 749,605	$ 749,605
Intangible assets	19,267
Goodwill	92,990
Purchase accounting adjustments to goodwill					$ (2,700)	$ 2,700			$ 146
Fair value of non-PCD loans	4,940,000
Fair value of PCD loans	841,200
Contractual balance of non-PCD loans	5,050,000
Contractual balance of PCD loans	896,500
ACL expense (reversal)	75,900							75,900
Initial ACL Recorded for PCD Loans	21,200
Merger-related expense							$ 5	$ 44,100
Opus Bank | Core Deposits
Business Acquisition [Line Items]
Intangible assets	16,100
Opus Bank | Customer Relationships
Business Acquisition [Line Items]
Intangible assets	$ 3,200
Opus Bank | Common Stock
Business Acquisition [Line Items]
Common stock issued as consideration (in shares) | shares	34,407,403
Common stock issued for tax withholding (in shares) | shares	165,136
Value of equity interests issued and issuable		747,458
Opus Bank | Equity warrants
Business Acquisition [Line Items]
Common stock issued as consideration (in shares) | shares	406,778
Value of equity interests issued and issuable	$ 1,800
Opus Bank | Equity Option
Business Acquisition [Line Items]
Common stock issued as consideration (in shares) | shares	9,538
Value of equity interests issued and issuable	$ 46
Opus Bank | Restricted Stock Awards
Business Acquisition [Line Items]
Value of equity interests issued and issuable	328	$ 328
Opus Bank | Opus Bank
Business Acquisition [Line Items]
Total assets	8,320,000
Total gross loans	5,940,000
Total deposits	$ 6,910,000
Number of banking offices | office	46