Loans and Leases - Nonaccrual and Performing Loans and Leases Held for Investment (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Financing Receivable, Impaired [Line Items]
Nonaccrual	$ 85,615	$ 92,353
Loans held for investment, net	19,026,200	18,846,872	$ 18,735,543
Performing
Financing Receivable, Impaired [Line Items]
Loans held for investment, net	18,940,585	18,754,519
Real Estate Mortgage
Financing Receivable, Impaired [Line Items]
Nonaccrual	47,128	20,824
Loans held for investment, net	7,877,045	7,972,747	7,896,924
Real Estate Mortgage | Performing
Financing Receivable, Impaired [Line Items]
Loans held for investment, net	7,829,917	7,951,923
Real Estate Mortgage | Commercial
Financing Receivable, Impaired [Line Items]
Nonaccrual	45,120	18,346
Loans held for investment, net	4,192,466	4,202,687
Real Estate Mortgage | Commercial | Performing
Financing Receivable, Impaired [Line Items]
Loans held for investment, net	4,147,346	4,184,341
Real Estate Mortgage | Residential Real Estate
Financing Receivable, Impaired [Line Items]
Nonaccrual	2,008	2,478
Loans held for investment, net	3,684,579	3,770,060
Real Estate Mortgage | Residential Real Estate | Performing
Financing Receivable, Impaired [Line Items]
Loans held for investment, net	3,682,571	3,767,582
Real Estate Construction and Land
Financing Receivable, Impaired [Line Items]
Nonaccrual	324	364
Loans held for investment, net	3,423,747	2,737,802	2,551,474
Real Estate Construction and Land | Performing
Financing Receivable, Impaired [Line Items]
Loans held for investment, net	3,423,423	2,737,438
Real Estate Construction and Land | Commercial Real Estate Construction Loan Receivable
Financing Receivable, Impaired [Line Items]
Nonaccrual	324	364
Loans held for investment, net	1,241,647	1,082,368
Real Estate Construction and Land | Commercial Real Estate Construction Loan Receivable | Performing
Financing Receivable, Impaired [Line Items]
Loans held for investment, net	1,241,323	1,082,004
Real Estate Construction and Land | Residential Real Estate Construction Financing Receivable
Financing Receivable, Impaired [Line Items]
Nonaccrual	0	0
Loans held for investment, net	2,182,100	1,655,434
Real Estate Construction and Land | Residential Real Estate Construction Financing Receivable | Performing
Financing Receivable, Impaired [Line Items]
Loans held for investment, net	2,182,100	1,655,434
Commercial
Financing Receivable, Impaired [Line Items]
Nonaccrual	37,759	70,672
Loans held for investment, net	7,363,174	7,695,496	7,878,557
Commercial | Performing
Financing Receivable, Impaired [Line Items]
Loans held for investment, net	7,325,415	7,624,824
Commercial | Asset-based
Financing Receivable, Impaired [Line Items]
Nonaccrual	2,817	30,162
Loans held for investment, net	3,153,048	3,748,407
Commercial | Asset-based | Performing
Financing Receivable, Impaired [Line Items]
Loans held for investment, net	3,150,231	3,718,245
Commercial | Venture Capital Loans
Financing Receivable, Impaired [Line Items]
Nonaccrual	2,001	12,916
Loans held for investment, net	1,637,132	2,179,422
Commercial | Venture Capital Loans | Performing
Financing Receivable, Impaired [Line Items]
Loans held for investment, net	1,635,131	2,166,506
Commercial | Other Commercial
Financing Receivable, Impaired [Line Items]
Nonaccrual	32,941	27,594
Loans held for investment, net	2,572,994	1,767,667
Commercial | Other Commercial | Performing
Financing Receivable, Impaired [Line Items]
Loans held for investment, net	2,540,053	1,740,073
Consumer
Financing Receivable, Impaired [Line Items]
Nonaccrual	404	493
Loans held for investment, net	362,234	440,827	$ 408,588
Consumer | Performing
Financing Receivable, Impaired [Line Items]
Loans held for investment, net	$ 361,830	$ 440,334