Acquisitions - Narrative (Details) $ / shares in Units, $ in Thousands				1 Months Ended	3 Months Ended	12 Months Ended
	Jun. 01, 2020 USD ($) office $ / shares shares	May 29, 2020 USD ($) $ / shares	Jul. 01, 2018 USD ($) $ / shares shares	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	May 31, 2020 USD ($) client_account	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Assets						$ 19,736,544	$ 11,776,012
Gross loans held for investment						13,236,433	8,722,311
Total deposits						16,214,177	8,898,509
Goodwill						898,569	808,322	$ 808,726			$ 493,329
Purchase accounting adjustments						(2,597)	(404)	2,354
ACL expense (reversal)						172,262	7,135	8,156
Initial ACL Recorded for PCD Loans						21,242
Goodwill acquired during the year						92,844	0	313,043
PENSCO
Business Acquisition [Line Items]
Custodial assets									$ 14,480,000
Number of client accounts | client_account									44,000
Opus
Business Acquisition [Line Items]
Equity issued, ratio	0.900
Stock transaction value (usd per share) | $ / shares	$ 19.31
Closing price for the Corporation's common stock (in dollars per share) | $ / shares		$ 21.62
Total merger consideration	$ 749,605	$ 749,605
Intangible assets	19,267
Goodwill	90,247
Purchase accounting adjustments				$ 2,600
Fair value of non-PCD loans	4,940,000
Aggregate fair value of PCD loans acquired	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
Initial ACL Recorded for PCD Loans	21,200
Merger-related expense						$ 49,100
Total assets acquired	8,012,034
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deposits	6,915,990
Opus | Core Deposits
Business Acquisition [Line Items]
Intangible assets	16,100
Opus | Customer Relationships
Business Acquisition [Line Items]
Intangible assets	$ 3,200
Opus | Common Stock
Business Acquisition [Line Items]
Number of shares of common stock issued as consideration (in shares) | shares	34,407,403
Common stock issued issued for tax withholding (in shares) | shares	165,136
Vaue of equity interests issued and issuable		747,458
Opus | Equity warrants
Business Acquisition [Line Items]
Number of shares of common stock issued as consideration (in shares) | shares	406,778
Vaue of equity interests issued and issuable	$ 1,800
Opus | Equity Option
Business Acquisition [Line Items]
Number of shares of common stock issued as consideration (in shares) | shares	9,538
Vaue of equity interests issued and issuable	$ 46
Opus | Restricted Stock
Business Acquisition [Line Items]
Vaue of equity interests issued and issuable	328	$ 328
Opus | Opus
Business Acquisition [Line Items]
Assets	8,320,000
Gross loans held for investment	5,940,000
Total deposits	$ 6,910,000
Number of banking offices | office	46
Grandpoint Capital, Inc.
Business Acquisition [Line Items]
Equity issued, ratio			0.4750
Total merger consideration			$ 602,152
Intangible assets			71,943
Goodwill			312,639
Purchase accounting adjustments					$ 580		$ (404)
Total assets acquired			3,076,224							$ 3,050,000
Gross loans acquired										2,400,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deposits			2,506,929							$ 2,510,000
Aggregate cash paid for shares and consideration payable			$ 28,100
Closing stock price of common stock (in dollars per share) | $ / shares			$ 38.15
Goodwill acquired during the year			$ 312,600					$ 313,000
Grandpoint Capital, Inc. | Common Stock
Business Acquisition [Line Items]
Number of shares of common stock issued as consideration (in shares) | shares			15,758,089