Investment Securities - Schedule of Securities with Gross Unrealized Losses Aggregated by Investment Category (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Gross Unrealized Losses
Less than twelve months	$ (30,200)	$ (44,874)
Over twelve months	(3,485)	(2,677)
Total	(33,685)	(47,551)
Estimated Fair Value
Less than twelve months	2,562,038	2,526,660
Over twelve months	171,114	102,762
Total	2,733,152	2,629,422
Gross Unrealized Losses
Less than twelve months	(257)	(463)
Over twelve months	(451)	(487)
Total	(708)	(950)
Estimated Fair Value
Less than twelve months	16,858	22,890
Over twelve months	10,308	10,960
Total	27,166	33,850
U.S. Government-Sponsored Enterprise Obligations
Gross Unrealized Losses
Less than twelve months	(317)	(549)
Over twelve months	0	0
Total	(317)	(549)
Estimated Fair Value
Less than twelve months	110,276	150,554
Over twelve months	0	0
Total	110,276	150,554
Obligations of State and Political Subdivisions
Gross Unrealized Losses
Less than twelve months	(1,580)	(5,207)
Over twelve months	(114)	0
Total	(1,694)	(5,207)
Estimated Fair Value
Less than twelve months	82,187	148,059
Over twelve months	4,541	0
Total	86,728	148,059
Mortgage-Backed Securities
Gross Unrealized Losses
Less than twelve months	(27,763)	(38,667)
Over twelve months	(3,355)	(2,624)
Total	(31,118)	(41,291)
Estimated Fair Value
Less than twelve months	2,330,909	2,191,563
Over twelve months	165,156	98,912
Total	2,496,065	2,290,475
Other Securities
Gross Unrealized Losses
Less than twelve months	(540)	(451)
Over twelve months	(16)	(53)
Total	(556)	(504)
Estimated Fair Value
Less than twelve months	38,666	36,484
Over twelve months	1,417	3,850
Total	40,083	40,334
Obligations of State and Political Subdivisions
Gross Unrealized Losses
Less than twelve months	(57)	(133)
Over twelve months	0	0
Total	(57)	(133)
Estimated Fair Value
Less than twelve months	5,014	10,602
Over twelve months	0	0
Total	5,014	10,602
Mortgage-Backed Securities
Gross Unrealized Losses
Less than twelve months	(200)	(330)
Over twelve months	(451)	(487)
Total	(651)	(817)
Estimated Fair Value
Less than twelve months	11,844	12,288
Over twelve months	10,308	10,960
Total	$ 22,152	$ 23,248