Investment Securities - Schedule of Securities with Gross Unrealized Losses Aggregated by Investment Category (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Gross Unrealized Losses
Less than twelve months	$ 0	$ (54)
Over twelve months	(220)	(823)
Total	(220)	(877)
Fair Value
Less than twelve months	0	5,529
Over twelve months	15,743	21,735
Total	15,743	27,264
Gross Unrealized Losses
Less than twelve months	(297)	(13,448)
Over twelve months	(564)	(5,173)
Total	(861)	(18,621)
Fair Value
Less than twelve months	52,401	1,515,493
Over twelve months	101,366	213,830
Total	153,767	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)
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)
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	(259)	(11,737)
Over twelve months	(533)	(4,786)
Total	(792)	(16,523)
Fair Value
Less than twelve months	41,403	1,279,914
Over twelve months	94,658	185,215
Total	136,061	1,465,129
Other Securities
Gross Unrealized Losses
Less than twelve months	(38)	(488)
Over twelve months	(31)	(9)
Total	(69)	(497)
Fair Value
Less than twelve months	10,998	51,975
Over twelve months	6,708	499
Total	17,706	52,474
Obligations of States and Political Subdivisions
Gross Unrealized Losses
Less than twelve months		(9)
Over twelve months		(92)
Total		(101)
Fair Value
Less than twelve months		1,999
Over twelve months		4,162
Total		6,161
Mortgage-Backed Securities
Gross Unrealized Losses
Less than twelve months	0	(45)
Over twelve months	(220)	(731)
Total	(220)	(776)
Fair Value
Less than twelve months	0	3,530
Over twelve months	15,743	17,573
Total	$ 15,743	$ 21,103