Loans Held for Investment and Allowance for Loan Losses - Allowance for Credit Losses (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2020	Sep. 30, 2019
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance		$ 195,047	$ 191,522
Provision for credit losses on loans		219,962	60,083
Charge-offs		135,615	62,874
Recoveries		2,186	1,407
Net charge-offs (recoveries)	$ 1,600	133,429	61,467
Ending balance	290,165	290,165	190,138
Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance		8,585
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance		102,254	96,814
Provision for credit losses on loans		47,263	30,309
Charge-offs		35,376	30,869
Recoveries		883	1,300
Net charge-offs (recoveries)		34,493	29,569
Ending balance	99,284	99,284	97,554
Commercial \| Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance		(15,740)
Energy
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance		60,253	34,882
Provision for credit losses on loans		127,470	42,243
Charge-offs		100,239	31,828
Recoveries		1,303	107
Net charge-offs (recoveries)		98,936	31,721
Ending balance	112,941	112,941	45,404
Energy \| Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance		24,154
Mortgage finance
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance		2,265	0
Provision for credit losses on loans		430	1,966
Charge-offs		0	0
Recoveries		0	0
Net charge-offs (recoveries)		0	0
Ending balance	4,726	4,726	1,966
Mortgage finance \| Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance		2,031
Real estate
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance		30,275	52,595
Provision for credit losses on loans		44,799	(7,204)
Charge-offs		0	177
Recoveries		0	0
Net charge-offs (recoveries)		0	177
Ending balance	73,214	73,214	45,214
Real estate \| Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance		(1,860)
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	$ 0
Additional Qualitative Reserve \| Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance		$ 0

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Amount of allowance for credit loss on financing receivable. Excludes allowance for financing receivable covered under loss sharing agreement.

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Amount of increase (decrease) in allowance for credit loss on financing receivable.

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Amount of increase in allowance for credit loss on financing receivable from recovery.

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Amount of expense related to estimated loss from loan and lease transactions.

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