Investment Securities - Schedule of Amortized Cost and Fair Values of Investment Securities, with Gross Unrealized Gains and Losses (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities
Amortized cost	$ 4,668,025	$ 4,647,931
Gross unrealized gain	1,624	5,972
Gross unrealized losses	(127,163)	(63,841)
Estimated fair value	4,542,486	4,590,062
Schedule of Held-to-maturity Securities
Amortized cost	224,241	227,318
Gross unrealized gains	761	1,571
Gross unrealized losses	(5,134)	(925)
Estimated fair value	219,868	227,964
Fair value of securities held as collateral	2,100,000	0
US Treasury Securities
Schedule of Available-for-sale Securities
Amortized cost	6,824
Gross unrealized gain	0
Gross unrealized losses	0
Estimated fair value	6,824
U.S. Government-Sponsored Enterprise Obligations
Schedule of Available-for-sale Securities
Amortized cost	41,001	41,003
Gross unrealized gain	11	18
Gross unrealized losses	(654)	(406)
Estimated fair value	40,358	40,615
Obligations of State and Political Subdivisions
Schedule of Available-for-sale Securities
Amortized cost	267,962	271,451
Gross unrealized gain	1,228	4,246
Gross unrealized losses	(4,156)	(1,493)
Estimated fair value	265,034	274,204
Schedule of Held-to-maturity Securities
Amortized cost	204,404	206,736
Gross unrealized gains	726	1,530
Gross unrealized losses	(4,120)	(275)
Estimated fair value	201,010	207,991
Mortgage-Backed Securities
Schedule of Available-for-sale Securities
Amortized cost	4,277,033	4,221,472
Gross unrealized gain	279	1,461
Gross unrealized losses	(121,215)	(61,028)
Estimated fair value	4,156,097	4,161,905
Schedule of Held-to-maturity Securities
Amortized cost	19,837	20,582
Gross unrealized gains	35	41
Gross unrealized losses	(1,014)	(650)
Estimated fair value	18,858	19,973
Other Securities
Schedule of Available-for-sale Securities
Amortized cost	75,205	114,005
Gross unrealized gain	106	247
Gross unrealized losses	(1,138)	(914)
Estimated fair value	$ 74,173	$ 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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