Securities - Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
Jun. 30, 2013	Dec. 31, 2012
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	$ 3,166,692	$ 2,956,381
Held to Maturity, Gross Unrealized Gains	31,611	45,556
Held to Maturity, Gross Unrealized Losses	127,954	4,993
Held to Maturity, Estimated Fair Value	3,070,349	2,996,944
Available for Sale, Amortized Cost	5,916,029	5,990,577
Available for Sale, Gross Unrealized Gains	124,535	212,747
Available for Sale, Gross Unrealized Losses	7,194	25
Available for Sale, Estimated Fair Value	6,033,370	6,203,299
U.S. Treasury [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	248,386	248,188
Held to Maturity, Gross Unrealized Gains	22,629	29,859
Held to Maturity, Gross Unrealized Losses
Held to Maturity, Estimated Fair Value	271,015	278,047
Available for Sale, Amortized Cost	3,021,080	3,020,115
Available for Sale, Gross Unrealized Gains	21,643	37,806
Available for Sale, Gross Unrealized Losses
Available for Sale, Estimated Fair Value	3,042,723	3,057,921
Residential mortgage- backed securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	10,262	10,725
Held to Maturity, Gross Unrealized Gains	118	300
Held to Maturity, Gross Unrealized Losses	109
Held to Maturity, Estimated Fair Value	10,271	11,025
Available for Sale, Amortized Cost	1,995,296	2,382,514
Available for Sale, Gross Unrealized Gains	81,366	135,514
Available for Sale, Gross Unrealized Losses	1,089	25
Available for Sale, Estimated Fair Value	2,075,573	2,518,003
States and political subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	2,907,044	2,696,468
Held to Maturity, Gross Unrealized Gains	8,864	15,397
Held to Maturity, Gross Unrealized Losses	127,843	4,993
Held to Maturity, Estimated Fair Value	2,788,065	2,706,872
Available for Sale, Amortized Cost	863,754	552,056
Available for Sale, Gross Unrealized Gains	21,526	39,427
Available for Sale, Gross Unrealized Losses	6,105
Available for Sale, Estimated Fair Value	879,175	591,483
Other [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	1,000	1,000
Held to Maturity, Gross Unrealized Gains
Held to Maturity, Gross Unrealized Losses	2
Held to Maturity, Estimated Fair Value	998	1,000
Available for Sale, Amortized Cost	35,899	35,892
Available for Sale, Gross Unrealized Gains
Available for Sale, Gross Unrealized Losses
Available for Sale, Estimated Fair Value	$ 35,899	$ 35,892

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

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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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-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

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Amount of unrealized gain before deducting unrealized loss on investments in debt and equity securities classified as available-for-sale securities.

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-Name Accounting Standards Codification

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Amount of unrealized loss before deducting unrealized gain on investments in available-for-sale securities.

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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 investments in debt securities classified as held-to-maturity.

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Amount of unrecognized gain before loss on investments in held-to-maturity securities.

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Amount of unrecognized loss before gain on investments in held-to-maturity securities. Excludes write-down for other than temporary impairment (OTTI).

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