Loans Held for Investment and Allowance for Loan Losses - Allowance for Credit Losses (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 195,047	$ 191,522
Provision for credit losses on loans	95,029	21,639
Charge-offs	58,383	4,865
Recoveries	680	277
Net charge-offs (recoveries)	57,703	4,588
Ending balance	240,958	208,573
Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	8,585
Ending balance
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	102,254	96,814
Provision for credit losses on loans	24,902	12,872
Charge-offs	20,653	4,865
Recoveries	257	277
Net charge-offs (recoveries)	20,396	4,588
Ending balance	91,020	105,098
Commercial | Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	(15,740)
Ending balance
Energy
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	60,253	34,882
Provision for credit losses on loans	66,821	12,387
Charge-offs	37,730	0
Recoveries	423	0
Net charge-offs (recoveries)	37,307	0
Ending balance	113,921	47,269
Energy | Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	24,154
Ending balance
Mortgage finance
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	2,265	0
Provision for credit losses on loans	35	1,300
Charge-offs	0	0
Recoveries	0	0
Net charge-offs (recoveries)	0	0
Ending balance	4,331	1,300
Mortgage finance | Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	2,031
Ending balance
Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	30,275	52,595
Provision for credit losses on loans	3,271	2,311
Charge-offs	0	0
Recoveries	0	0
Net charge-offs (recoveries)	0	0
Ending balance	31,686	54,906
Real Estate | Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	(1,860)
Ending balance
Additional Qualitative Reserve
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	0	7,231
Provision for credit losses on loans	0	(7,231)
Charge-offs	0	0
Recoveries	0	0
Net charge-offs (recoveries)	0	0
Ending balance	0	$ 0
Additional Qualitative Reserve | Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 0
Ending balance