Loans and Leases - Nonaccrual and Performing Loans and Leases Held for Investment (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Financing Receivable, Impaired [Line Items]
Nonaccrual	$ 67,652	$ 91,163	$ 95,602
Loans held for investment, net	18,979,228	19,083,377	19,745,305
Real Estate Mortgage
Financing Receivable, Impaired [Line Items]
Nonaccrual	48,907	45,557	21,396
Loans held for investment, net	7,987,213	7,899,936	8,008,944
Real Estate Construction and Land
Financing Receivable, Impaired [Line Items]
Nonaccrual	718	315	351
Loans held for investment, net	3,565,823	3,360,281	2,880,253
Commercial
Financing Receivable, Impaired [Line Items]
Nonaccrual	17,471	44,902	73,442
Loans held for investment, net	7,085,840	7,502,905	8,426,224
Consumer
Financing Receivable, Impaired [Line Items]
Nonaccrual	556	389	413
Loans held for investment, net	340,352	320,255	$ 429,884
Performing
Financing Receivable, Impaired [Line Items]
Loans held for investment, net	18,911,576	18,992,214
Performing | Real Estate Mortgage
Financing Receivable, Impaired [Line Items]
Loans held for investment, net	7,938,306	7,854,379
Performing | Real Estate Construction and Land
Financing Receivable, Impaired [Line Items]
Loans held for investment, net	3,565,105	3,359,966
Performing | Commercial
Financing Receivable, Impaired [Line Items]
Loans held for investment, net	7,068,369	7,458,003
Performing | Consumer
Financing Receivable, Impaired [Line Items]
Loans held for investment, net	339,796	319,866
Asset-based | Commercial
Financing Receivable, Impaired [Line Items]
Nonaccrual	2,379	2,679
Loans held for investment, net	3,383,403	3,429,283
Asset-based | Performing | Commercial
Financing Receivable, Impaired [Line Items]
Loans held for investment, net	3,381,024	3,426,604
Venture Capital Loans | Commercial
Financing Receivable, Impaired [Line Items]
Nonaccrual	2,432	1,980
Loans held for investment, net	1,495,798	1,698,508
Venture Capital Loans | Performing | Commercial
Financing Receivable, Impaired [Line Items]
Loans held for investment, net	1,493,366	1,696,528
Other Commercial | Commercial
Financing Receivable, Impaired [Line Items]
Nonaccrual	12,660	40,243
Loans held for investment, net	2,206,639	2,375,114
Other Commercial | Performing | Commercial
Financing Receivable, Impaired [Line Items]
Loans held for investment, net	2,193,979	2,334,871
Commercial Real Estate Construction Loan Receivable | Real Estate Construction and Land
Financing Receivable, Impaired [Line Items]
Nonaccrual	302	315
Loans held for investment, net	990,035	1,117,121
Commercial Real Estate Construction Loan Receivable | Performing | Real Estate Construction and Land
Financing Receivable, Impaired [Line Items]
Loans held for investment, net	989,733	1,116,806
Residential Real Estate Construction Financing Receivable | Real Estate Construction and Land
Financing Receivable, Impaired [Line Items]
Nonaccrual	416	0
Loans held for investment, net	2,575,788	2,243,160
Residential Real Estate Construction Financing Receivable | Performing | Real Estate Construction and Land
Financing Receivable, Impaired [Line Items]
Loans held for investment, net	2,575,372	2,243,160
Commercial | Real Estate Mortgage
Financing Receivable, Impaired [Line Items]
Nonaccrual	46,436	43,731
Loans held for investment, net	3,941,610	4,096,671
Commercial | Performing | Real Estate Mortgage
Financing Receivable, Impaired [Line Items]
Loans held for investment, net	3,895,174	4,052,940
Residential Real Estate | Real Estate Mortgage
Financing Receivable, Impaired [Line Items]
Nonaccrual	2,471	1,826
Loans held for investment, net	4,045,603	3,803,265
Residential Real Estate | Performing | Real Estate Mortgage
Financing Receivable, Impaired [Line Items]
Loans held for investment, net	$ 4,043,132	$ 3,801,439