Acquisitions - Narrative (Details) $ / shares in Units, $ in Thousands			3 Months Ended			6 Months Ended
	Jun. 01, 2020 USD ($) office $ / shares shares	May 29, 2020 USD ($) $ / shares	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	May 31, 2020 USD ($) client_account	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Business Acquisition [Line Items]
Total assets			$ 20,517,074			$ 20,517,074			$ 11,776,012
Total gross loans			15,082,884			15,082,884			8,722,311
Total deposits			16,976,693			16,976,693			8,898,509
Goodwill			901,166	$ 901,166	$ 808,322	901,166	$ 808,322		$ 808,322	$ 808,726
ACL expense (reversal)			150,257		742	175,639	2,754
Initial ACL Recorded for PCD Loans			21,242			21,242
Merger-related expense			39,346	$ 1,724	$ 5	41,070	$ 660
Common Stock
Business Acquisition [Line Items]
Vaue of equity interests issued and issuable		$ 747,458
Equity warrants
Business Acquisition [Line Items]
Vaue of equity interests issued and issuable	$ 1,800
Equity Option
Business Acquisition [Line Items]
Vaue of equity interests issued and issuable	46
Restricted Stock Awards
Business Acquisition [Line Items]
Vaue of equity interests issued and issuable	$ 328	$ 328
PENSCO
Business Acquisition [Line Items]
Custodial assets								$ 14,480,000
Number of client accounts | client_account								44,000
Opus Bank
Business Acquisition [Line Items]
Equity issued, ratio	0.900
Stock transaction value (usd per share) | $ / shares	$ 19.31
Closing price of Corporation's common stock (in dollars per share) | $ / shares		$ 21.62
Total merger consideration	$ 749,605
Goodwill	92,800
Fair value of non-PCD loans	4,940,000
Fair value of PCD loans	841,234
Contractual balance of non-PCD loans	5,050,000
Contractual balance of PCD loans	896,483
ACL expense (reversal)	75,900		$ 75,900			$ 75,900
Initial ACL Recorded for PCD Loans	21,200
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 issued for tax withholding (in shares) | shares	165,136
Opus Bank | Equity warrants
Business Acquisition [Line Items]
Common stock issued as consideration (in shares) | shares	406,778
Opus Bank | Equity Option
Business Acquisition [Line Items]
Common stock issued as consideration (in shares) | shares	9,538
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