Fair Value Measurements - OREO (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016	Sep. 13, 2013
Fair Value
Non-covered impaired loans	$ 6,737,417		$ 6,737,417			$ 6,212,712
Covered impaired loans	142,737		142,737			179,400
Covered other real estate owned	29,856	$ 40,343	29,856	$ 40,343	$ 34,895	36,744	$ 42,304	$ 51,642
Total Gains (Losses) of impaired loans	371	(1,260)	1,838	(8,818)
Covered
Fair Value
Covered impaired loans	142,737		142,737			179,400
Total Gains (Losses) of impaired loans	678	(781)	1,382	(2,237)
Noncovered
Fair Value
Non-covered impaired loans	6,737,417		6,737,417			6,212,712
Total Gains (Losses) of impaired loans	(307)	(479)	456	(6,581)
Estimate of Fair Value
Fair Value
Non-covered impaired loans	6,833,717		6,833,717			6,406,757
Covered impaired loans	217,503		217,503			269,386
Level 2 | Estimate of Fair Value
Fair Value
Non-covered impaired loans	593,276		593,276			577,889
Level 2 | Estimate of Fair Value | Covered
Fair Value
Covered other real estate owned	29,900		29,900			36,700
Level 2 | Estimate of Fair Value | Noncovered
Fair Value
Non-covered other real estate owned	2,700		2,700			3,900
Level 3 | Estimate of Fair Value
Fair Value
Non-covered impaired loans	6,240,441		6,240,441			5,828,868
Covered impaired loans	217,503		217,503			$ 269,386
Nonrecurring | Estimate of Fair Value | Covered
Fair Value
Covered impaired loans	63,194		63,194
Covered other real estate owned	19,555		19,555
Total Gains (Losses) of impaired loans	683	(787)	1,429	(1,764)
Total Gains (Losses) of other real estate owned	(303)	(388)	(2,027)	(2,523)
Nonrecurring | Estimate of Fair Value | Noncovered
Fair Value
Non-covered impaired loans	17,749		17,749
Non-covered other real estate owned	529		529
Total Gains (Losses) of impaired loans	(15)	793	(1,486)	323
Total Gains (Losses) of other real estate owned	(91)	$ (135)	(167)	$ (258)
Nonrecurring | Level 2 | FNB | Covered | PlainsCapital (the Bank)
Fair value measurements
Acquired OREO									$ 135,200
Nonrecurring | Level 2 | Estimate of Fair Value | Covered
Fair Value
Covered other real estate owned	19,555		19,555
Nonrecurring | Level 2 | Estimate of Fair Value | Noncovered
Fair Value
Non-covered other real estate owned	529		529
Nonrecurring | Level 3 | Estimate of Fair Value | Covered
Fair Value
Covered impaired loans	63,194		63,194
Nonrecurring | Level 3 | Estimate of Fair Value | Noncovered
Fair Value
Non-covered impaired loans	$ 17,749		$ 17,749