Investment Securities - Schedule of Securities with Gross Unrealized Losses Aggregated by Investment Category (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Gross Unrealized Losses
Less than twelve months	$ 0	$ (54)
Over twelve months	(175)	(823)
Total	(175)	(877)
Fair Value
Less than twelve months	2,212	5,529
Over twelve months	11,698	21,735
Total	13,910	27,264
Gross Unrealized Losses
Less than twelve months	(1,021)	(13,448)
Over twelve months	(899)	(5,173)
Total	(1,920)	(18,621)
Fair Value
Less than twelve months	267,529	1,515,493
Over twelve months	98,175	213,830
Total	365,704	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	(127)	(9)
Over twelve months	0	0
Total	(127)	(9)
Fair Value
Less than twelve months	30,222	5,765
Over twelve months	0	0
Total	30,222	5,765
Mortgage-Backed Securities
Gross Unrealized Losses
Less than twelve months	(893)	(11,737)
Over twelve months	(850)	(4,786)
Total	(1,743)	(16,523)
Fair Value
Less than twelve months	236,800	1,279,914
Over twelve months	94,259	185,215
Total	331,059	1,465,129
Other Securities
Gross Unrealized Losses
Less than twelve months	(1)	(488)
Over twelve months	(49)	(9)
Total	(50)	(497)
Fair Value
Less than twelve months	507	51,975
Over twelve months	3,916	499
Total	4,423	52,474
Obligations of States and Political Subdivisions
Gross Unrealized Losses
Less than twelve months	0	(9)
Over twelve months	0	(92)
Total	0	(101)
Fair Value
Less than twelve months	2,212	1,999
Over twelve months	0	4,162
Total	2,212	6,161
Mortgage-Backed Securities
Gross Unrealized Losses
Less than twelve months	0	(45)
Over twelve months	(175)	(731)
Total	(175)	(776)
Fair Value
Less than twelve months	0	3,530
Over twelve months	11,698	17,573
Total	$ 11,698	$ 21,103