Loans Held for Investment and Allowance for Loan Losses - Age Analysis (Details) - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2019	Jan. 01, 2020
Financing Receivable, Past Due [Line Items]
Total Past Due	$ 69,194,000
Non-accrual	174,031,000		$ 225,400,000
Current	25,366,660,000
Total	25,609,885,000	$ 24,736,642,000
Non-accrual With No Allowance	94,624,000
Premium finance loans past due and still accruing	14,800,000
Nonaccrual, interest income	0	0
Interest income reversed	708,000
30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	34,005,000
60 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	14,110,000
Greater Than 90 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	21,079,000
Commercial
Financing Receivable, Past Due [Line Items]
Total Past Due	44,773,000
Non-accrual	51,206,000		88,600,000
Current	9,068,682,000
Total	9,164,661,000	9,133,444,000	10,230,828,000
Non-accrual With No Allowance	21,146,000
Commercial | 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	19,451,000
Commercial | 60 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	6,383,000
Commercial | Greater Than 90 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	18,939,000
Energy
Financing Receivable, Past Due [Line Items]
Total Past Due	295,000
Non-accrual	103,874,000		125,000,000.0
Current	1,041,995,000
Total	1,146,164,000	1,425,309,000
Non-accrual With No Allowance	54,986,000
Energy | 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	295,000
Energy | 60 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	0
Energy | Greater Than 90 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	0
Mortgage finance
Financing Receivable, Past Due [Line Items]
Total Past Due	0
Non-accrual	0
Current	8,972,626,000
Total	8,972,626,000	8,169,849,000	8,169,849,000
Non-accrual With No Allowance	0
Mortgage finance | 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	0
Mortgage finance | 60 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	0
Mortgage finance | Greater Than 90 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	0
Real Estate
Financing Receivable, Past Due [Line Items]
Total	6,326,434,000	$ 6,008,040,000	3,444,701,000
Real Estate | Commercial Real Estate
Financing Receivable, Past Due [Line Items]
Total Past Due	14,588,000
Non-accrual	241,000		9,400,000
Current	3,983,297,000
Total	3,998,126,000
Non-accrual With No Allowance	0
Real Estate | Residential Homebuilder Finance
Financing Receivable, Past Due [Line Items]
Total Past Due	0
Non-accrual	0
Current	1,032,974,000
Total	1,032,974,000
Non-accrual With No Allowance	0
Real Estate | Other Financing Receivable
Financing Receivable, Past Due [Line Items]
Total Past Due	6,591,000
Non-accrual	18,492,000		881,000
Current	911,511,000
Total	936,594,000
Non-accrual With No Allowance	18,492,000
Real Estate | Secured by 1-4 family
Financing Receivable, Past Due [Line Items]
Total Past Due	2,947,000
Non-accrual	218,000		$ 1,400,000
Current	355,575,000
Total	358,740,000
Non-accrual With No Allowance	0
Real Estate | 30 to 59 Days Past Due | Commercial Real Estate
Financing Receivable, Past Due [Line Items]
Total Past Due	13,307,000
Real Estate | 30 to 59 Days Past Due | Residential Homebuilder Finance
Financing Receivable, Past Due [Line Items]
Total Past Due	0
Real Estate | 30 to 59 Days Past Due | Other Financing Receivable
Financing Receivable, Past Due [Line Items]
Total Past Due	581,000
Real Estate | 30 to 59 Days Past Due | Secured by 1-4 family
Financing Receivable, Past Due [Line Items]
Total Past Due	371,000
Real Estate | 60 to 89 Days Past Due | Commercial Real Estate
Financing Receivable, Past Due [Line Items]
Total Past Due	31,000
Real Estate | 60 to 89 Days Past Due | Residential Homebuilder Finance
Financing Receivable, Past Due [Line Items]
Total Past Due	0
Real Estate | 60 to 89 Days Past Due | Other Financing Receivable
Financing Receivable, Past Due [Line Items]
Total Past Due	6,010,000
Real Estate | 60 to 89 Days Past Due | Secured by 1-4 family
Financing Receivable, Past Due [Line Items]
Total Past Due	1,686,000
Real Estate | Greater Than 90 Days | Commercial Real Estate
Financing Receivable, Past Due [Line Items]
Total Past Due	1,250,000
Real Estate | Greater Than 90 Days | Residential Homebuilder Finance
Financing Receivable, Past Due [Line Items]
Total Past Due	0
Real Estate | Greater Than 90 Days | Other Financing Receivable
Financing Receivable, Past Due [Line Items]
Total Past Due	0
Real Estate | Greater Than 90 Days | Secured by 1-4 family
Financing Receivable, Past Due [Line Items]
Total Past Due	$ 890,000