Investment Securities - Schedule of Securities with Gross Unrealized Losses Aggregated by Investment Category (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Gross Unrealized Losses
Less than twelve months	$ (16)	$ (54)
Over twelve months	(404)	(823)
Total	(420)	(877)
Estimated Fair Value
Less than twelve months	884	5,529
Over twelve months	20,609	21,735
Total	21,493	27,264
Gross Unrealized Losses
Less than twelve months	(889)	(13,448)
Over twelve months	(1,709)	(5,173)
Total	(2,598)	(18,621)
Estimated Fair Value
Less than twelve months	135,313	1,515,493
Over twelve months	186,906	213,830
Total	322,219	1,729,323
U.S. Government-Sponsored Enterprise Obligations
Gross Unrealized Losses
Less than twelve months		(1,214)
Over twelve months		(378)
Total		(1,592)
Estimated Fair Value
Less than twelve months		177,839
Over twelve months		28,116
Total		205,955
Obligations of States and Political Subdivisions
Gross Unrealized Losses
Less than twelve months		(9)
Over twelve months		0
Total		(9)
Estimated Fair Value
Less than twelve months		5,765
Over twelve months		0
Total		5,765
Mortgage-Backed Securities
Gross Unrealized Losses
Less than twelve months	(447)	(11,737)
Over twelve months	(1,707)	(4,786)
Total	(2,154)	(16,523)
Estimated Fair Value
Less than twelve months	105,507	1,279,914
Over twelve months	186,400	185,215
Total	291,907	1,465,129
Other Securities
Gross Unrealized Losses
Less than twelve months	(442)	(488)
Over twelve months	(2)	(9)
Total	(444)	(497)
Estimated Fair Value
Less than twelve months	29,806	51,975
Over twelve months	506	499
Total	30,312	52,474
Obligations of States and Political Subdivisions
Gross Unrealized Losses
Less than twelve months	(16)	(9)
Over twelve months	(21)	(92)
Total	(37)	(101)
Estimated Fair Value
Less than twelve months	884	1,999
Over twelve months	2,219	4,162
Total	3,103	6,161
Mortgage-Backed Securities
Gross Unrealized Losses
Less than twelve months	0	(45)
Over twelve months	(383)	(731)
Total	(383)	(776)
Estimated Fair Value
Less than twelve months	0	3,530
Over twelve months	18,390	17,573
Total	$ 18,390	$ 21,103