Segment Information - Summary of Segment Reporting Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Net interest income	$ 149,122	$ 136,245	$ 293,165	$ 243,865
Provision for loans losses	6,204	8,251	11,382	14,150
Other income	36,007	40,506	67,199	60,652
Other expense	93,410	112,137	183,862	174,979
Income taxes	19,241	19,304	37,140	39,520
Net income	66,274	37,059	127,980	75,868
Total assets (liabilities)	19,207,603	19,035,600	19,207,603	19,035,600
Average assets (liabilities)	18,712,384	18,183,015	18,628,621	16,304,164
Operating Segments [Member] | Community Banking [Member]
Segment Reporting Information [Line Items]
Net interest income	149,448	138,498	295,070	248,158
Provision for loans losses	6,204	8,251	11,382	14,150
Other income	17,465	18,223	35,235	35,036
Other expense	76,495	78,631	148,986	141,383
Income taxes	18,948	23,920	38,224	43,724
Net income	65,266	45,919	131,713	83,937
Total assets (liabilities)	19,112,133	18,984,921	19,112,133	18,984,921
Average assets (liabilities)	18,658,168	18,070,305	18,583,975	16,415,261
Operating Segments [Member] | Mortgage Banking [Member]
Segment Reporting Information [Line Items]
Net interest income	264	90	640	90
Provision for loans losses	0	0	0	0
Other income	23,468	22,393	38,351	22,393
Other expense	21,225	18,708	39,609	18,708
Income taxes	564	1,293	(139)	1,293
Net income	1,943	2,482	(479)	2,482
Total assets (liabilities)	388,292	382,086	388,292	382,086
Average assets (liabilities)	335,781	236,074	274,211	118,689
Other [Member]
Segment Reporting Information [Line Items]
Net interest income	(3,032)	(2,343)	(5,626)	(4,383)
Provision for loans losses	0	0	0	0
Other income	68	(110)	(754)	3,223
Other expense	(1,758)	14,798	(2,181)	14,888
Income taxes	(271)	(5,909)	(945)	(5,497)
Net income	(935)	(11,342)	(3,254)	(10,551)
Total assets (liabilities)	10,895	(4,807)	10,895	(4,807)
Average assets (liabilities)	6,776	(20,180)	9,231	(24,551)
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Net interest income	2,442	0	3,081	0
Provision for loans losses	0	0	0	0
Other income	(4,994)	0	(5,633)	0
Other expense	(2,552)	0	(2,552)	0
Income taxes	0	0	0	0
Net income	0	0	0	0
Total assets (liabilities)	(303,717)	(326,600)	(303,717)	(326,600)
Average assets (liabilities)	$ (288,341)	$ (103,184)	$ (238,796)	$ (205,235)