Assets Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading securities	$ 39,919	$ 58,142
Available for sale securities	6,487,124	6,107,882
Company-owned life insurance	10,482
Total	6,537,525	6,166,024
Deferred compensation	10,527
Contingent consideration	52,087	72,046
Total	62,614
U.S. Treasury
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading securities	400	400
Available for sale securities	118,517	189,325
U.S. Agencies
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading securities	754	1,517
Available for sale securities	817,524	1,632,009
Mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading securities	8,294	29,641
Available for sale securities	3,436,623	2,492,348
State and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading securities	5,284	7,252
Available for sale securities	1,883,727	1,694,036
Trading - other
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading securities	25,187	19,332
Corporates
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale securities	230,733	100,164
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading securities	26,036	21,234
Available for sale securities	1,166,774	1,921,498
Total	1,192,810	1,942,732
Deferred compensation	10,527
Total	10,527
Quoted Prices in Active Markets for Identical Assets (Level 1) \| U.S. Treasury
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading securities	400	400
Available for sale securities	118,517	189,325
Quoted Prices in Active Markets for Identical Assets (Level 1) \| U.S. Agencies
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading securities	754	1,517
Available for sale securities	817,524	1,632,009
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Trading - other
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading securities	24,882	19,317
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Corporates
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale securities	230,733	100,164
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading securities	13,883	36,908
Available for sale securities	5,320,350	4,186,384
Company-owned life insurance	10,482
Total	5,344,715	4,223,292
Significant Other Observable Inputs (Level 2) \| Mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading securities	8,294	29,641
Available for sale securities	3,436,623	2,492,348
Significant Other Observable Inputs (Level 2) \| State and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading securities	5,284	7,252
Available for sale securities	1,883,727	1,694,036
Significant Other Observable Inputs (Level 2) \| Trading - other
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading securities	305	15
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Contingent consideration	52,087	72,046
Total	$ 52,087

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

This item represents the amount that could be realized under a life insurance contract or contracts owned by the Entity as of the date of the statement of financial position. Such Entity-owned life insurance policies are commonly known as corporate-owned life insurance (COLI) or bank-owned life insurance (BOLI).

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- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Trading Securities which consist of all investments in certain debt and equity securities that are bought and held principally for the purpose of selling them in the near term (thus held for only a short period of time) or for which an entity has elected the fair value option. Trading generally reflects active and frequent buying and selling, and trading securities are generally used with the objective of generating profits on short-term differences in price, but this category of investments may also include items which are not intended to be sold in the near term or for which the fair value option has been elected.

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