Loans and Leases - Credit Risk Rating Categories for Loans and Leases Held for Investment (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	$ 19,026,200	$ 18,846,872	$ 18,735,543
Classified
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	274,572	175,912
Special Mention
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	783,756	322,956
Pass
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	17,967,872	18,348,004
Real Estate Mortgage
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	7,877,045	7,972,747	7,896,924
Real Estate Mortgage | Classified
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	119,209	42,135
Real Estate Mortgage | Special Mention
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	393,745	31,781
Real Estate Mortgage | Pass
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	7,364,091	7,898,831
Real Estate Mortgage | Commercial
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	4,192,466	4,202,687
Real Estate Mortgage | Commercial | Classified
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	109,651	33,535
Real Estate Mortgage | Commercial | Special Mention
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	330,367	30,070
Real Estate Mortgage | Commercial | Pass
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	3,752,448	4,139,082
Real Estate Mortgage | Residential Real Estate
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	3,684,579	3,770,060
Real Estate Mortgage | Residential Real Estate | Classified
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	9,558	8,600
Real Estate Mortgage | Residential Real Estate | Special Mention
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	63,378	1,711
Real Estate Mortgage | Residential Real Estate | Pass
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	3,611,643	3,759,749
Real Estate Construction and Land
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	3,423,747	2,737,802	2,551,474
Real Estate Construction and Land | Classified
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	324	364
Real Estate Construction and Land | Special Mention
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	13,838	1,429
Real Estate Construction and Land | Pass
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	3,409,585	2,736,009
Real Estate Construction and Land | Commercial Real Estate Construction Loan Receivable
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	1,241,647	1,082,368
Real Estate Construction and Land | Commercial Real Estate Construction Loan Receivable | Classified
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	324	364
Real Estate Construction and Land | Commercial Real Estate Construction Loan Receivable | Special Mention
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	13,838	0
Real Estate Construction and Land | Commercial Real Estate Construction Loan Receivable | Pass
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	1,227,485	1,082,004
Real Estate Construction and Land | Residential Real Estate Construction Financing Receivable
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	2,182,100	1,655,434
Real Estate Construction and Land | Residential Real Estate Construction Financing Receivable | Classified
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	0	0
Real Estate Construction and Land | Residential Real Estate Construction Financing Receivable | Special Mention
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	0	1,429
Real Estate Construction and Land | Residential Real Estate Construction Financing Receivable | Pass
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	2,182,100	1,654,005
Commercial
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	7,363,174	7,695,496	7,878,557
Commercial | Classified
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	154,531	132,800
Commercial | Special Mention
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	371,655	288,534
Commercial | Pass
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	6,836,988	7,274,162
Commercial | Asset-based
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	3,153,048	3,748,407
Commercial | Asset-based | Classified
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	27,900	32,223
Commercial | Asset-based | Special Mention
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	190,193	38,936
Commercial | Asset-based | Pass
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	2,934,955	3,677,248
Commercial | Venture Capital Loans
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	1,637,132	2,179,422
Commercial | Venture Capital Loans | Classified
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	15,078	35,316
Commercial | Venture Capital Loans | Special Mention
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	123,675	74,813
Commercial | Venture Capital Loans | Pass
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	1,498,379	2,069,293
Commercial | Other Commercial
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	2,572,994	1,767,667
Commercial | Other Commercial | Classified
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	111,553	65,261
Commercial | Other Commercial | Special Mention
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	57,787	174,785
Commercial | Other Commercial | Pass
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	2,403,654	1,527,621
Consumer
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	362,234	440,827	$ 408,588
Consumer | Classified
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	508	613
Consumer | Special Mention
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	4,518	1,212
Consumer | Pass
Financing Receivable, Past Due [Line Items]
Loans held for investment, net	$ 357,208	439,002
Non Purchased Credit Impaired Loans and Leases
Financing Receivable, Past Due [Line Items]
Loans held for investment, net		$ 18,846,872