Securities - Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities, Presented by Contractual Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Held to Maturity, Due in one year or less, Amortized Cost	$ 56,511
Held to Maturity, Due after one year through five years, Amortized Cost	131,987
Held to Maturity, Due after five years through ten years, Amortized Cost	492,460
Held to Maturity, Due after ten years, Amortized Cost	427,764
Held to Maturity, Residential mortgage-backed securities, Amortized Cost	3,040
Held-to-maturity Securities, Equity Securities, Amortized cost	0
Held to Maturity, Amortized Cost	1,111,762	$ 1,432,098
Held to Maturity, Due in one year or less, Estimated Fair Value	57,181
Held to Maturity, Due after one year through five years, Estimated Fair Value	133,521
Held to Maturity, Due after five years through ten years, Estimated Fair Value	492,354
Held to Maturity, Due after ten years, Estimated Fair Value	430,835
Held to Maturity, Residential mortgage-backed securities, Estimated Fair Value	2,954
Held-to-maturity Securities, Equity Securities, Fair Value	0
Held to Maturity, Estimated Fair Value	1,116,845	1,455,791
Available-for-sale Securities, Due in one year or less, Amortized Cost	858,235
Available for Sale, Due after one year through five years, Amortized Cost	3,243,532
Available for Sale, Due after five years through ten years, Amortized Cost	489,960
Available for Sale, Due after ten years, Amortized Cost	5,915,328
Available for Sale, Residential mortgage-backed securities, Amortized Cost	756,649
Available-for-sale Securities, Equity Securities, Amortized cost	42,651
Available for Sale, Amortized Cost	11,306,355	10,329,951
Available for Sale, Due in one year or less, Estimated Fair Value	851,163
Available for Sale, Due after one year through five years, Estimated Fair Value	3,204,804
Available for Sale, Due after five years through ten years, Estimated Fair Value	487,536
Available for Sale, Due after ten years, Estimated Fair Value	5,836,066
Available for Sale, Residential mortgage-backed securities, Estimated Fair Value	756,371
Available For Sale Securities Equity Securities Fair Value	42,651
Available for Sale, Estimated Fair Value	$ 11,178,591	$ 10,489,009

Available For Sale Securities Equity Securities Fair Value

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Mortgage-backed securities Available For Sale, Amortized Cost

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Mortgage Backed Securities Held To Maturity Net Carrying Amount

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

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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Amount after adjustments of available-for-sale debt securities at cost, maturing in the sixth fiscal year through the tenth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of available-for-sale debt securities maturing in the sixth fiscal year through the tenth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount after adjustments of available-for-sale debt securities at cost, maturing in the second fiscal year through the fifth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of available-for-sale debt securities maturing in the second fiscal year through the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount after adjustments of available-for-sale debt securities at cost, maturing after the tenth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of available-for-sale debt securities maturing after the tenth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount after adjustments of available-for-sale debt securities at cost, maturing in the next fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of available-for-sale debt securities maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of equity securities categorized neither as held-to-maturity nor as trading.

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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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Fair value of held-to-maturity debt securities maturing in the sixth fiscal year through the tenth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Carrying amount of debt securities accounted for as held-to-maturity, net of adjustments including, but not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized, and fair value hedge accounting adjustments, maturing in the fifth through tenth fiscal year following the latest fiscal year.

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Fair value of held-to-maturity debt securities maturing in the second fiscal year through the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Carrying amount of debt securities accounted for as held-to-maturity, net of adjustments including, but not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized, and fair value hedge accounting adjustments, maturing in the next fiscal year through the fifth fiscal year following the latest fiscal year.

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Fair value of held-to-maturity debt securities maturing after the tenth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Carrying amount of debt securities accounted for as held-to-maturity, net of adjustments including, but not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized, and fair value hedge accounting adjustments, maturing after the tenth fiscal year following the latest fiscal year.

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Fair value of held-to-maturity debt securities maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of held-to-maturity debt securities at fair value that do not have a single maturity date and which the entity has decided to disclose separately rather than allocating the fair value over several maturity groupings.

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Fair value of investments in debt securities classified as held-to-maturity.

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Fair value of securitized, pay-through debt securities collateralized by real estate mortgage loans classified as available-for-sale.

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Fair value of securitized, pay-through debt securities collateralized by real estate mortgage loans classified as held-to-maturity.

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