Loans Held for Investment and Allowance for Loan Losses (Details 7) $ in Thousands	Dec. 31, 2019 USD ($)
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 47,709
Non-accrual	225,384
Current	24,463,549
Total	24,736,642
Premium finance loans past due and still accruing	8,500
Commercial | Business loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	35,330
Non-accrual	88,411
Current	8,681,989
Total	8,805,730
Commercial | Energy loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0
Non-accrual	125,049
Current	1,300,049
Total	1,425,098
Mortgage Finance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0
Non-accrual	0
Current	8,169,849
Total	8,169,849
Construction | Market risk
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0
Non-accrual	0
Current	2,457,986
Total	2,457,986
Construction | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0
Non-accrual	0
Current	93,764
Total	93,764
Construction | Secured by 1-4 family
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0
Non-accrual	0
Current	11,589
Total	11,589
Real estate | Market risk
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	10,786
Non-accrual	9,370
Current	2,238,384
Total	2,258,540
Real estate | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	688
Non-accrual	881
Current	810,149
Total	811,718
Real estate | Secured by 1-4 family
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	389
Non-accrual	1,449
Current	372,605
Total	374,443
Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	212
Non-accrual	34
Current	71,217
Total	71,463
Equipment leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	304
Non-accrual	190
Current	255,968
Total	256,462
30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	19,940
30 to 59 Days Past Due | Commercial | Business loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	8,746
30 to 59 Days Past Due | Commercial | Energy loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0
30 to 59 Days Past Due | Mortgage Finance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0
30 to 59 Days Past Due | Construction | Market risk
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0
30 to 59 Days Past Due | Construction | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0
30 to 59 Days Past Due | Construction | Secured by 1-4 family
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0
30 to 59 Days Past Due | Real estate | Market risk
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	10,786
30 to 59 Days Past Due | Real estate | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0
30 to 59 Days Past Due | Real estate | Secured by 1-4 family
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	104
30 to 59 Days Past Due | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0
30 to 59 Days Past Due | Equipment leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	304
60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	10,185
60 to 89 Days Past Due | Commercial | Business loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	9,299
60 to 89 Days Past Due | Commercial | Energy loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0
60 to 89 Days Past Due | Mortgage Finance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0
60 to 89 Days Past Due | Construction | Market risk
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0
60 to 89 Days Past Due | Construction | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0
60 to 89 Days Past Due | Construction | Secured by 1-4 family
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0
60 to 89 Days Past Due | Real estate | Market risk
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0
60 to 89 Days Past Due | Real estate | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	495
60 to 89 Days Past Due | Real estate | Secured by 1-4 family
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	179
60 to 89 Days Past Due | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	212
60 to 89 Days Past Due | Equipment leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0
Greater Than 90 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	17,584
Greater Than 90 Days | Commercial | Business loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	17,285
Greater Than 90 Days | Commercial | Energy loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0
Greater Than 90 Days | Mortgage Finance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0
Greater Than 90 Days | Construction | Market risk
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0
Greater Than 90 Days | Construction | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0
Greater Than 90 Days | Construction | Secured by 1-4 family
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0
Greater Than 90 Days | Real estate | Market risk
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0
Greater Than 90 Days | Real estate | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	193
Greater Than 90 Days | Real estate | Secured by 1-4 family
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	106
Greater Than 90 Days | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0
Greater Than 90 Days | Equipment leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 0