Non-Covered Loans and Allowance for Non-Covered Loan Losses - TDRs (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018 loan item	Mar. 31, 2017 USD ($) item	Mar. 31, 2018 USD ($) item	Mar. 31, 2017 USD ($) item
AB Note | Minimum | PlainsCapital
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 | loan	0
Noncovered | Payment Term Extension
TDRs, Non-covered loans
Number of TDR loans granted | item		2
TDR at extension		$ 2,838
Total Modifications		$ 2,812
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			$ 655	$ 1,481
TDR with payment at least 30 days past due			$ 595	$ 1,470
Noncovered | Secured | Payment Term Extension
TDRs, Non-covered loans
Number of TDR loans granted | item		1
TDR at extension		$ 1,357
Total Modifications		$ 1,342
Noncovered | Secured by Commercial Properties | Payment Term Extension
TDRs, Non-covered loans
Number of TDR loans granted | item		1
TDR at extension		$ 1,481
Total Modifications		$ 1,470
Number of TDRs granted in preceding twelve months for which payment was at least 30 days past due | item				1
TDR modifications, in which a payment was at least 30 days past due				$ 1,481
TDR with payment at least 30 days past due				$ 1,470
Noncovered | Commercial construction loans and land development | Payment Term Extension
TDRs, Non-covered loans
Number of TDRs granted in preceding twelve months for which payment was at least 30 days past due | item			1
TDR modifications, in which a payment was at least 30 days past due			$ 655
TDR with payment at least 30 days past due			$ 595