Loans and Leases - Aging Analysis of Loans and Leases Held for Investment (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Financing Receivable, Past Due [Line Items]
Total Past Due	$ 50,551	$ 48,755
30-89 Days Past Due	17,128	12,232
90 or More Days Past Due	33,423	36,523
Current	18,928,677	19,034,622
Loans held for investment, net	18,979,228	19,083,377	$ 19,745,305
Real Estate Mortgage
Financing Receivable, Past Due [Line Items]
Total Past Due	33,423	36,868
30-89 Days Past Due	6,538	7,350
90 or More Days Past Due	26,885	29,518
Current	7,953,790	7,863,068
Loans held for investment, net	7,987,213	7,899,936	8,008,944
Real Estate Construction and Land
Financing Receivable, Past Due [Line Items]
Total Past Due	1,657	759
30-89 Days Past Due	1,241	759
90 or More Days Past Due	416	0
Current	3,564,166	3,359,522
Loans held for investment, net	3,565,823	3,360,281	2,880,253
Commercial
Financing Receivable, Past Due [Line Items]
Total Past Due	14,229	9,757
30-89 Days Past Due	8,395	2,863
90 or More Days Past Due	5,834	6,894
Current	7,071,611	7,493,148
Loans held for investment, net	7,085,840	7,502,905	8,426,224
Consumer
Financing Receivable, Past Due [Line Items]
Total Past Due	1,242	1,371
30-89 Days Past Due	954	1,260
90 or More Days Past Due	288	111
Current	339,110	318,884
Loans held for investment, net	340,352	320,255	$ 429,884
Asset-based | Commercial
Financing Receivable, Past Due [Line Items]
Total Past Due	1,880	2,128
30-89 Days Past Due	0	0
90 or More Days Past Due	1,880	2,128
Current	3,381,523	3,427,155
Loans held for investment, net	3,383,403	3,429,283
Venture Capital Loans | Commercial
Financing Receivable, Past Due [Line Items]
Total Past Due	6,750	540
30-89 Days Past Due	6,750	540
90 or More Days Past Due	0	0
Current	1,489,048	1,697,968
Loans held for investment, net	1,495,798	1,698,508
Other Commercial | Commercial
Financing Receivable, Past Due [Line Items]
Total Past Due	5,599	7,089
30-89 Days Past Due	1,645	2,323
90 or More Days Past Due	3,954	4,766
Current	2,201,040	2,368,025
Loans held for investment, net	2,206,639	2,375,114
Commercial Real Estate Construction Loan Receivable | Real Estate Construction and Land
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
30-89 Days Past Due	0	0
90 or More Days Past Due	0	0
Current	990,035	1,117,121
Loans held for investment, net	990,035	1,117,121
Residential Real Estate Construction Financing Receivable | Real Estate Construction and Land
Financing Receivable, Past Due [Line Items]
Total Past Due	1,657	759
30-89 Days Past Due	1,241	759
90 or More Days Past Due	416	0
Current	2,574,131	2,242,401
Loans held for investment, net	2,575,788	2,243,160
Commercial | Real Estate Mortgage
Financing Receivable, Past Due [Line Items]
Total Past Due	25,869	35,895
30-89 Days Past Due	36	6,750
90 or More Days Past Due	25,833	29,145
Current	3,915,741	4,060,776
Loans held for investment, net	3,941,610	4,096,671
Residential Real Estate | Real Estate Mortgage
Financing Receivable, Past Due [Line Items]
Total Past Due	7,554	973
30-89 Days Past Due	6,502	600
90 or More Days Past Due	1,052	373
Current	4,038,049	3,802,292
Loans held for investment, net	$ 4,045,603	$ 3,803,265