Investment Securities - Schedule of Amortized Cost and Fair Values of Investment Securities, with Gross Unrealized Gains and Losses (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule of Held-to-maturity Securities
Amortized cost	$ 90,653	$ 98,928
Gross unrealized gains	2,932	2,910
Gross unrealized losses	(175)	(877)
Estimated fair value	93,410	100,961
Schedule of Available-for-sale Securities
Amortized cost	2,845,969	2,803,430
Gross Unrealized Gains	41,364	15,477
Gross Unrealized Losses	(1,920)	(18,621)
Securities available for sale	2,885,413	2,800,286
U.S. Government-Sponsored Enterprise Obligations
Schedule of Available-for-sale Securities
Amortized cost	217,859	252,514
Gross Unrealized Gains	2,613	1,161
Gross Unrealized Losses	0	(1,592)
Securities available for sale	220,472	252,083
Obligations of States and Political Subdivisions
Schedule of Available-for-sale Securities
Amortized cost	245,086	182,541
Gross Unrealized Gains	7,946	5,429
Gross Unrealized Losses	(127)	(9)
Securities available for sale	252,905	187,961
Mortgage-Backed Securities
Schedule of Available-for-sale Securities
Amortized cost	2,283,336	2,272,879
Gross Unrealized Gains	29,475	8,457
Gross Unrealized Losses	(1,743)	(16,523)
Securities available for sale	2,311,068	2,264,813
Other Securities
Schedule of Available-for-sale Securities
Amortized cost	99,688	95,496
Gross Unrealized Gains	1,330	430
Gross Unrealized Losses	(50)	(497)
Securities available for sale	100,968	95,429
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities
Amortized cost	64,898	69,979
Gross unrealized gains	2,727	2,803
Gross unrealized losses	0	(101)
Estimated fair value	67,625	72,681
Mortgage-Backed Securities
Schedule of Held-to-maturity Securities
Amortized cost	25,755	28,949
Gross unrealized gains	205	107
Gross unrealized losses	(175)	(776)
Estimated fair value	$ 25,785	$ 28,280

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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Amount of accumulated unrealized gain on investments in debt securities classified as held-to-maturity.

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