Fair Value Measurements - OREO (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015	Sep. 13, 2013
Fair Value
Non-covered impaired loans	$ 6,090,034		$ 6,090,034			$ 5,789,313
Covered impaired loans	188,269		188,269			255,714
Covered other real estate owned	40,343	$ 61,988	40,343	$ 61,988	$ 42,304	51,642	$ 67,634	$ 99,090
Total Gains (Losses) of impaired loans	(1,260)	(3,990)	(8,818)	(36,273)
Covered
Fair Value
Covered impaired loans	188,269		188,269			255,714
Total Gains (Losses) of impaired loans	(781)	729	(2,237)	825
Noncovered
Fair Value
Non-covered impaired loans	6,090,034		6,090,034			5,789,313
Total Gains (Losses) of impaired loans	(479)	(4,719)	(6,581)	(37,098)
Estimate of Fair Value
Fair Value
Non-covered impaired loans	6,288,495		6,288,495			5,962,052
Covered impaired loans	284,424		284,424			367,444
Estimate of Fair Value | Covered
Fair Value
Covered other real estate owned	40,300		40,300			51,600
Estimate of Fair Value | Noncovered | Other Assets
Fair Value
Non-covered other real estate owned	4,800		4,800			4,500
Level 2 | Estimate of Fair Value
Fair Value
Non-covered impaired loans	485,604		485,604			502,077
Level 3 | Estimate of Fair Value
Fair Value
Non-covered impaired loans	5,802,891		5,802,891			5,459,975
Covered impaired loans	284,424		284,424			$ 367,444
Nonrecurring | Estimate of Fair Value | Covered
Fair Value
Covered impaired loans	58,490		58,490
Covered other real estate owned	6,969		6,969
Total Gains (Losses) of impaired loans	(787)	725	(1,764)	815
Total Gains (Losses) of other real estate owned	(388)	(2,552)	(2,523)	(14,284)
Nonrecurring | Estimate of Fair Value | Noncovered
Fair Value
Non-covered impaired loans	42,953		42,953
Non-covered other real estate owned	3,512		3,512
Total Gains (Losses) of impaired loans	793	1,149	323	1,284
Total Gains (Losses) of other real estate owned	(135)	$ (427)	(258)	$ (439)
Nonrecurring | Level 2 | Estimate of Fair Value | Covered
Fair Value
Covered other real estate owned	6,969		6,969
Nonrecurring | Level 2 | Estimate of Fair Value | Noncovered
Fair Value
Non-covered other real estate owned	3,512		3,512
Nonrecurring | Level 3 | FNB | Covered | PlainsCapital
Fair value measurements
Acquired OREO									$ 135,200
Nonrecurring | Level 3 | Estimate of Fair Value | Covered
Fair Value
Covered impaired loans	58,490		58,490
Nonrecurring | Level 3 | Estimate of Fair Value | Noncovered
Fair Value
Non-covered impaired loans	$ 42,953		$ 42,953