Non-Covered Loans and Allowance for Non-Covered Loan Losses - TDRs (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018 item	Sep. 30, 2017 item	Sep. 30, 2018 USD ($) item	Sep. 30, 2017 USD ($) loan item
AB Note | Minimum | PlainsCapital (the Bank)
TDRs, Non-covered loans
Number of loans into which a single loan may be reconfigured | item			2
Noncovered
TDRs, Non-covered loans
Number of TDR loans granted | item	0	0	0
Noncovered | Payment Term Extension
TDRs, Non-covered loans
Number of TDR loans granted | item				3
TDR at extension				$ 3,493
Total Modifications				$ 3,246
Number of TDRs granted in preceding twelve months for which payment was at least 30 days past due | item			1	1
TDR modifications, in which a payment was at least 30 days past due			$ 3,294	$ 1,481
TDR with payment at least 30 days past due			$ 3,130	$ 1,385
Noncovered | Secured | Payment Term Extension
TDRs, Non-covered loans
Number of TDR loans granted | loan				1
TDR at extension				$ 1,357
Total Modifications				$ 1,235
Noncovered | Secured by Commercial Properties | Payment Term Extension
TDRs, Non-covered loans
Number of TDR loans granted | loan				1
TDR at extension				$ 1,481
Total Modifications				$ 1,385
Number of TDRs granted in preceding twelve months for which payment was at least 30 days past due | item			1	1
TDR modifications, in which a payment was at least 30 days past due			$ 3,294	$ 1,481
TDR with payment at least 30 days past due			$ 3,130	$ 1,385
Noncovered | Commercial construction loans and land development | Payment Term Extension
TDRs, Non-covered loans
Number of TDR loans granted | loan				1
TDR at extension				$ 655
Total Modifications				$ 626