Parent Company Information - Schedule of Condensed Statements of Income (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income
Gains on securities transactions, net									$ (150)	$ (2,342)	$ (20)
Losses on sales of assets, net	$ 1,336	$ (159)	$ (564)	$ 77,720	$ (280)	$ (1,899)	$ (125)	$ (97)	78,333	(2,401)	(95)
Income tax expense (benefit)	36,967	25,307	27,532	57,196	18,074	18,046	18,961	13,184	147,002	68,265	90,831
Net Income	38,104	81,891	76,468	113,330	77,102	69,559	72,802	41,965	309,793	261,428	161,907
Dividends on preferred stock	3,172	3,172	3,172	3,172	3,172	3,172	3,172	3,172	12,688	12,688	9,449
Net Income Available to Common Shareholders	$ 34,932	$ 78,719	$ 73,296	$ 110,158	$ 73,930	$ 66,387	$ 69,630	$ 38,793	297,105	248,740	152,458
Parent Company
Income
Dividends from subsidiary									160,000	155,000	122,000
Income from subsidiary									4,550	4,550	4,550
Gains on securities transactions, net									0	3	0
Losses on sales of assets, net									0	(147)	0
Other interest and income									51	39	135
Total Income									164,601	159,445	126,685
Total Expenses									27,998	32,269	39,621
Income before income tax and equity in undistributed earnings of subsidiary									136,603	127,176	87,064
Income tax expense (benefit)									24,524	(20,547)	(30,179)
Income before equity in undistributed earnings of subsidiary									112,079	147,723	117,243
Equity in undistributed earnings of subsidiary									197,714	113,705	44,664
Net Income									309,793	261,428	161,907
Dividends on preferred stock									12,688	12,688	9,449
Net Income Available to Common Shareholders									$ 297,105	$ 248,740	$ 152,458