Investment Securities - Schedule of Amortized Cost and Fair Values of Investment Securities, with Gross Unrealized Gains and Losses (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities
Amortized cost	$ 4,797,640	$ 4,647,931
Gross unrealized gain	1,723	5,972
Gross unrealized losses	(148,448)	(63,841)
Estimated fair value	4,650,915	4,590,062
Schedule of Held-to-maturity Securities
Amortized cost	221,030	227,318
Gross unrealized gains	601	1,571
Gross unrealized losses	(5,322)	(925)
Estimated fair value	216,309	227,964
Fair value of securities held as collateral	2,300,000	2,100,000
U.S. Government-Sponsored Enterprise Obligations
Schedule of Available-for-sale Securities
Amortized cost	40,999	41,003
Gross unrealized gain	7	18
Gross unrealized losses	(715)	(406)
Estimated fair value	40,291	40,615
Obligations of State and Political Subdivisions
Schedule of Available-for-sale Securities
Amortized cost	263,524	271,451
Gross unrealized gain	1,246	4,246
Gross unrealized losses	(3,918)	(1,493)
Estimated fair value	260,852	274,204
Schedule of Held-to-maturity Securities
Amortized cost	201,472	206,736
Gross unrealized gains	569	1,530
Gross unrealized losses	(4,118)	(275)
Estimated fair value	197,923	207,991
Mortgage-Backed Securities
Schedule of Available-for-sale Securities
Amortized cost	4,370,968	4,221,472
Gross unrealized gain	448	1,461
Gross unrealized losses	(141,570)	(61,028)
Estimated fair value	4,229,846	4,161,905
Schedule of Held-to-maturity Securities
Amortized cost	19,558	20,582
Gross unrealized gains	32	41
Gross unrealized losses	(1,204)	(650)
Estimated fair value	18,386	19,973
Other Securities
Schedule of Available-for-sale Securities
Amortized cost	122,149	114,005
Gross unrealized gain	22	247
Gross unrealized losses	(2,245)	(914)
Estimated fair value	$ 119,926	$ 113,338

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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The carrying amount (fair value) as of the balance sheet date of securities held that had been pledged by counterparties under financing and lending arrangements, pursuant to which the entity has the right by agreement or custom to sell or re-pledge such securities.

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