Loans Held for Investment and Allowance for Loan Losses - Age Analysis (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019	Jan. 01, 2020
Financing Receivable, Past Due [Line Items]
Total Past Due	$ 99,792,000
Non-accrual	161,946,000		$ 225,400,000
Current	24,984,948,000
Total	25,246,686,000	$ 24,736,642,000
Non-accrual With No Allowance	69,281,000
Premium finance loans past due and still accruing	11,900,000
Nonaccrual, interest income	0	0
Interest income reversed	1,000,000.0
30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	72,130,000
60 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	11,766,000
Greater Than 90 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	15,896,000
Commercial
Financing Receivable, Past Due [Line Items]
Total Past Due	39,285,000
Non-accrual	62,511,000		88,600,000
Current	8,685,121,000
Total	8,786,917,000	9,133,444,000	10,230,828,000
Non-accrual With No Allowance	19,367,000
Commercial | 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	25,387,000
Commercial | 60 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	1,650,000
Commercial | Greater Than 90 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	12,248,000
Energy
Financing Receivable, Past Due [Line Items]
Total Past Due	22,665,000
Non-accrual	73,811,000		125,000,000.0
Current	872,517,000
Total	968,993,000	1,425,309,000
Non-accrual With No Allowance	25,090,000
Energy | 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	20,670,000
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	1,995,000
Mortgage finance
Financing Receivable, Past Due [Line Items]
Total Past Due	0
Non-accrual	0
Current	9,378,104,000
Total	9,378,104,000	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	6,112,672,000	$ 6,008,040,000	3,444,701,000
Real estate | Commercial Real Estate
Financing Receivable, Past Due [Line Items]
Total Past Due	35,027,000
Non-accrual	4,265,000		9,400,000
Current	3,821,307,000
Total	3,860,599,000
Non-accrual With No Allowance	4,028,000
Real estate | Residential Homebuilder Finance
Financing Receivable, Past Due [Line Items]
Total Past Due	0
Non-accrual	0
Current	971,775,000
Total	971,775,000
Non-accrual With No Allowance	0
Real estate | Other Financing Receivable
Financing Receivable, Past Due [Line Items]
Total Past Due	1,018,000
Non-accrual	21,141,000		881,000
Current	912,568,000
Total	934,727,000
Non-accrual With No Allowance	20,796,000
Real estate | Secured by 1-4 family
Financing Receivable, Past Due [Line Items]
Total Past Due	1,797,000
Non-accrual	218,000		$ 1,400,000
Current	343,556,000
Total	345,571,000
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	24,158,000
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	1,018,000
Real estate | 30 to 59 Days Past Due | Secured by 1-4 family
Financing Receivable, Past Due [Line Items]
Total Past Due	897,000
Real estate | 60 to 89 Days Past Due | Commercial Real Estate
Financing Receivable, Past Due [Line Items]
Total Past Due	9,619,000
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	0
Real estate | 60 to 89 Days Past Due | Secured by 1-4 family
Financing Receivable, Past Due [Line Items]
Total Past Due	497,000
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	$ 403,000