Loans Held for Investment - TDRs (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020 USD ($) loan	Jun. 30, 2019 USD ($) loan	Jun. 30, 2020 USD ($) loan	Jun. 30, 2019 USD ($) loan	Jun. 30, 2020 USD ($)
TDRs, Non-covered loans
COVID-19 related loan modifications	$ 968,100,000		$ 968,100,000		$ 968,100,000
Unadvanced commitments to borrowers	$ 100,000		$ 100,000		$ 100,000
Number of TDRs granted in preceding twelve months for which payment was at least 30 days past due			0	0	1
TDR modifications, in which a payment was at least 30 days past due					$ 1,625,000
TDR with payment at least 30 days past due					$ 1,421,000
AB Note | Minimum | PlainsCapital (the Bank)
TDRs, Non-covered loans
Number of loans into which a single loan may be reconfigured | loan			2
Payment Term Extension
TDRs, Non-covered loans
Number of TDR loans granted	1,000	3,000	2,000	3,000
TDR at extension	$ 6,750,000	$ 7,993,000	$ 7,839,000	$ 7,993,000
TDR modifications, in which a payment was at least 30 days past due	$ 2,000,000	$ 7,973,000	$ 3,166,000	$ 7,973,000
Commercial | Commercial And Industrial
TDRs, Non-covered loans
Number of TDRs granted in preceding twelve months for which payment was at least 30 days past due					1
TDR modifications, in which a payment was at least 30 days past due					$ 1,625,000
TDR with payment at least 30 days past due					$ 1,421,000
Commercial | Commercial And Industrial | Payment Term Extension
TDRs, Non-covered loans
Number of TDR loans granted | loan	1	3	2	3
TDR at extension	$ 6,750,000	$ 7,993,000	$ 7,839,000	$ 7,993,000
TDR modifications, in which a payment was at least 30 days past due	$ 2,000,000	$ 7,973,000	$ 3,166,000	$ 7,973,000