Parent Company Information - Schedule of Condensed Statements of Income (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income
Gains on securities transactions, net									$ (2,342)	$ (20)	$ 777
Losses on sales of assets, net									(2,402)	(95)	1,358
Income tax benefit	$ 18,074	$ 18,046	$ 18,961	$ 13,184	$ 34,958	$ 17,088	$ 20,714	$ 18,071	68,265	90,831	65,234
Net Income	77,102	69,559	72,802	41,965	26,098	39,649	50,065	46,095	261,428	161,907	168,146
Dividends on preferred stock	3,172	3,172	3,172	3,172	3,172	2,683	1,797	1,797	12,688	9,449	7,188
Net Income Available to Common Shareholders	$ 73,930	$ 66,387	$ 69,630	$ 38,793	$ 22,926	$ 36,966	$ 48,268	$ 44,298	248,740	152,458	160,958
Parent Company
Income
Dividends from subsidiary									155,000	122,000	90,000
Income from subsidiary									4,550	4,550	4,550
Gains on securities transactions, net									3	0	239
Losses on sales of assets, net									(147)	0	0
Other interest and income									39	135	34
Total Income									159,445	126,685	94,823
Total Expenses									32,269	39,621	33,604
Income before income tax and equity in undistributed earnings of subsidiary									127,176	87,064	61,219
Income tax benefit									(20,547)	(30,179)	(23,349)
Income before equity in undistributed earnings of subsidiary									147,723	117,243	84,568
Equity in undistributed earnings of subsidiary									113,705	44,664	83,578
Net Income									261,428	161,907	168,146
Dividends on preferred stock									12,688	9,449	7,188
Net Income Available to Common Shareholders									$ 248,740	$ 152,458	$ 160,958