Parent Company Information - Schedule of Condensed Statements of Income (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income
Gains (losses) on securities transactions, net									$ (20)	$ 777	$ 2,487
Income tax benefit	$ 34,958	$ 17,088	$ 20,714	$ 18,071	$ 18,336	$ 17,049	$ 15,460	$ 14,389	90,831	65,234	23,938
Net Income	26,098	39,649	50,065	46,095	50,090	42,842	39,027	36,187	161,907	168,146	102,957
Dividends on preferred stock	3,172	2,683	1,797	1,797	1,797	1,797	1,797	1,797	9,449	7,188	3,813
Net Income Available to Common Shareholders	$ 22,926	$ 36,966	$ 48,268	$ 44,298	$ 48,293	$ 41,045	$ 37,230	$ 34,390	152,458	160,958	99,144
Parent Company
Income
Dividends from subsidiary									122,000	90,000	110,000
Income from subsidiary									4,550	4,550	2,363
Gains (losses) on securities transactions, net									0	239	0
Other interest and income									135	34	211
Total Income									126,685	94,823	112,574
Total Expenses									39,621	33,604	20,578
Income before income tax benefit and equity in undistributed earnings (losses) of subsidiary									87,064	61,219	91,996
Income tax benefit									(30,179)	(23,349)	(21,939)
Income before equity in undistributed earnings (losses) of subsidiary									117,243	84,568	113,935
Equity in undistributed earnings (losses) of subsidiary									44,664	83,578	(10,978)
Net Income									161,907	168,146	102,957
Dividends on preferred stock									9,449	7,188	3,813
Net Income Available to Common Shareholders									$ 152,458	$ 160,958	$ 99,144