Loans and Leases - Nonaccrual and Performing Loans and Leases Held for Investment (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Financing Receivable, Impaired [Line Items]
Nonaccrual	$ 166,113	$ 92,353
Loans held for investment, net	19,694,631	18,846,872	$ 18,472,852
Performing
Financing Receivable, Impaired [Line Items]
Loans held for investment, net	19,528,518	18,754,519
Real Estate Mortgage
Financing Receivable, Impaired [Line Items]
Nonaccrual	63,978	20,824
Loans held for investment, net	7,955,734	7,972,747	8,076,026
Real Estate Mortgage | Performing
Financing Receivable, Impaired [Line Items]
Loans held for investment, net	7,891,756	7,951,923
Real Estate Mortgage | Commercial
Financing Receivable, Impaired [Line Items]
Nonaccrual	61,771	18,346
Loans held for investment, net	4,222,075	4,202,687
Real Estate Mortgage | Commercial | Performing
Financing Receivable, Impaired [Line Items]
Loans held for investment, net	4,160,304	4,184,341
Real Estate Mortgage | Residential Real Estate
Financing Receivable, Impaired [Line Items]
Nonaccrual	2,207	2,478
Loans held for investment, net	3,733,659	3,770,060
Real Estate Mortgage | Residential Real Estate | Performing
Financing Receivable, Impaired [Line Items]
Loans held for investment, net	3,731,452	3,767,582
Real Estate Construction and Land
Financing Receivable, Impaired [Line Items]
Nonaccrual	337	364
Loans held for investment, net	3,340,528	2,737,802	2,376,130
Real Estate Construction and Land | Performing
Financing Receivable, Impaired [Line Items]
Loans held for investment, net	3,340,191	2,737,438
Real Estate Construction and Land | Commercial Real Estate Construction Loan Receivable
Financing Receivable, Impaired [Line Items]
Nonaccrual	337	364
Loans held for investment, net	1,167,609	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	1,167,272	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	2,172,919	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	2,172,919	1,655,434
Commercial
Financing Receivable, Impaired [Line Items]
Nonaccrual	101,278	70,672
Loans held for investment, net	7,987,050	7,695,496	7,574,314
Commercial | Performing
Financing Receivable, Impaired [Line Items]
Loans held for investment, net	7,885,772	7,624,824
Commercial | Asset-based
Financing Receivable, Impaired [Line Items]
Nonaccrual	19,013	30,162
Loans held for investment, net	3,412,431	3,748,407
Commercial | Asset-based | Performing
Financing Receivable, Impaired [Line Items]
Loans held for investment, net	3,393,418	3,718,245
Commercial | Venture Capital Loans
Financing Receivable, Impaired [Line Items]
Nonaccrual	8,270	12,916
Loans held for investment, net	1,814,341	2,179,422
Commercial | Venture Capital Loans | Performing
Financing Receivable, Impaired [Line Items]
Loans held for investment, net	1,806,071	2,166,506
Commercial | Other Commercial
Financing Receivable, Impaired [Line Items]
Nonaccrual	73,995	27,594
Loans held for investment, net	2,760,278	1,767,667
Commercial | Other Commercial | Performing
Financing Receivable, Impaired [Line Items]
Loans held for investment, net	2,686,283	1,740,073
Consumer
Financing Receivable, Impaired [Line Items]
Nonaccrual	520	493
Loans held for investment, net	411,319	440,827	$ 446,382
Consumer | Performing
Financing Receivable, Impaired [Line Items]
Loans held for investment, net	$ 410,799	$ 440,334