Commitments and Contingencies (Details)				3 Months Ended
	May 30, 2017 USD ($) $ / shares shares	Jan. 01, 2015 $ / shares	Sep. 13, 2013 USD ($) item	Mar. 31, 2018 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Reserve for Indemnification Liability:
FDIC, collections received				$ 145,800,000
SWS
Commitments and Contingencies
Conversion of common stock		0.2496
Consideration paid in cash | $ / shares		$ 1.94
Fair value acquisition price (in dollars per share) | $ / shares		$ 6.92
Representation and Warranty Claims
Roll-forward of claims activity for loans put-back to the mortgage origination segment
Balance, beginning of period				33,702,000	$ 39,245,000	$ 40,669,000
Claims made				6,989,000		8,379,000
Claims resolved with no payment				(5,861,000)		(8,098,000)
Repurchases				(2,089,000)		(1,461,000)
Indemnification payments				(420,000)		(244,000)
Balance, end of period				32,321,000		39,245,000
Reserve for Indemnification Liability:
Total				33,702,000	39,245,000	40,669,000	$ 32,321,000	$ 33,702,000
Indemnification Agreement
Commitments and Contingencies
Provision for indemnification losses				700,000		800,000
Roll-forward of claims activity for loans put-back to the mortgage origination segment
Balance, beginning of period				23,472,000	18,952,000	18,239,000
Additions for new sales				728,000		847,000
Repurchases				(160,000)		(102,000)
Early payment defaults				(146,000)		(69,000)
Indemnification payments				(117,000)		(42,000)
Change in reserves for loans sold in prior years				(445,000)		79,000
Balance, end of period				23,332,000		18,952,000
Reserve for Indemnification Liability:
Specific claims							652,000	646,000
Incurred but not reported claims							22,680,000	22,826,000
Total				23,472,000	18,952,000	$ 18,239,000	23,332,000	$ 23,472,000
Shareholder Lawsuits | SWS
Commitments and Contingencies
Number of shares of SWS common stock held by purported shareholders | shares	7,438,453
Consideration paid in cash | $ / shares	$ 6.38
Fair value acquisition price (in dollars per share) | $ / shares	$ 6.38
Stock released from escrow and returned to unissued shares of common stock	$ 1,856,638
Pre-tax net increase to noninterest income					$ 11,600,000
PlainsCapital | FNB | Covered
Reserve for Indemnification Liability:
Number of loss-sharing agreements | item			2
Loans and OREO acquired			$ 1,200,000,000
Percentage of net losses to be absorbed by FDIC on the first $240.4 million of net losses incurred as per the loss sharing agreement			80.00%
Threshold amount of net losses incurred for 80% of net losses to be absorbed by FDIC as per the loss sharing agreement, first layer			$ 240,400,000
Percentage of net losses to be absorbed by FDIC in excess of $240.4 million up to and including $365.7 million of net losses incurred as per the loss sharing agreement			0.00%
Threshold amount of net losses incurred for 0% of net losses to be absorbed by FDIC as per the loss sharing agreement, second layer			$ 365,700,000
Percentage of net losses to be absorbed by FDIC in excess of $365.7 million of net losses incurred as per the loss sharing agreement			80.00%
Threshold limit of subsequent recoveries reimbursable to the FDIC under the loss share agreement			$ 0
Period for which payment is required to be made to the FDIC of true-up amount			10 years
Payment accrual based on the current aggregate estimate of realized losses							$ 16,500,000
Percentage of initial estimate of losses under loss sharing agreement			80.00%
Amount of initial estimate of losses under loss sharing agreement			$ 240,400,000
Covered net losses billed to the FDIC				$ 184,700,000
Percentage of covered net losses reimbursable under the loss-share agreements				80.00%
Amount of covered net losses reimbursable under the loss-share agreements				$ 147,800,000
FDIC, collections received				145,800,000
PlainsCapital | FNB | Covered | Commercial Loan
Reserve for Indemnification Liability:
Period of loss-sharing agreements in effect			5 years
Period of loss recovery provisions in effect			8 years
PlainsCapital | FNB | Covered | Single Family Residential Loans
Reserve for Indemnification Liability:
Period of loss-sharing agreements in effect			10 years
Period of loss recovery provisions in effect			10 years
PlainsCapital | FNB | Maximum | Covered
Reserve for Indemnification Liability:
Covered losses and reimbursable expenses				$ 240,400,000