Loans Held for Investment and Allowance for Loan Losses - Age Analysis (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Jan. 01, 2020
Financing Receivable, Past Due [Line Items]
Total Past Due	$ 82,718,000
Non-accrual	219,165,000		$ 225,400,000
Current	24,217,312,000
Total	24,519,195,000	$ 24,736,642,000
Non-accrual With No Allowance	62,513,000
Premium finance loans past due and still accruing	8,600,000
Nonaccrual, interest income	0	0
Interest income reversed	393,000
30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	51,788,000
60 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	9,656,000
Greater Than 90 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	21,274,000
Commercial
Financing Receivable, Past Due [Line Items]
Total Past Due	55,042,000
Non-accrual	61,953,000		88,600,000
Current	9,285,255,000
Total	9,402,250,000	9,133,444,000	10,230,828,000
Non-accrual With No Allowance	16,903,000
Commercial | 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	26,013,000
Commercial | 60 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	9,656,000
Commercial | Greater Than 90 Days
Financing Receivable, Past Due [Line Items]
Total Past Due	19,373,000
Energy
Financing Receivable, Past Due [Line Items]
Total Past Due	0
Non-accrual	151,858,000		125,000,000.0
Current	1,179,631,000
Total	1,331,489,000	1,425,309,000
Non-accrual With No Allowance	40,719,000
Energy | 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	0
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	7,588,803,000
Total	7,588,803,000	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	6,196,653,000	$ 6,008,040,000	3,444,701,000
Real Estate | Commercial Real Estate
Financing Receivable, Past Due [Line Items]
Total Past Due	11,138,000
Non-accrual	244,000		9,400,000
Current	3,759,800,000
Total	3,771,182,000
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	1,101,493,000
Total	1,101,493,000
Non-accrual With No Allowance	0
Real Estate | Other Financing Receivable
Financing Receivable, Past Due [Line Items]
Total Past Due	16,395,000
Non-accrual	3,689,000		881,000
Current	925,789,000
Total	945,873,000
Non-accrual With No Allowance	3,689,000
Real Estate | Secured by 1-4 family
Financing Receivable, Past Due [Line Items]
Total Past Due	143,000
Non-accrual	1,421,000		$ 1,400,000
Current	376,541,000
Total	378,105,000
Non-accrual With No Allowance	1,202,000
Real Estate | 30 to 59 Days Past Due | Commercial Real Estate
Financing Receivable, Past Due [Line Items]
Total Past Due	9,301,000
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	16,395,000
Real Estate | 30 to 59 Days Past Due | Secured by 1-4 family
Financing Receivable, Past Due [Line Items]
Total Past Due	79,000
Real Estate | 60 to 89 Days Past Due | Commercial Real Estate
Financing Receivable, Past Due [Line Items]
Total Past Due	0
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	0
Real Estate | Greater Than 90 Days | Commercial Real Estate
Financing Receivable, Past Due [Line Items]
Total Past Due	1,837,000
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	$ 64,000