Available for Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 4,724,137	$ 4,180,376
Gross Unrealized Gains	100,881	92,236
Gross Unrealized Losses	18,964	19,357
Fair Value	4,806,054	4,253,255
Tax exempt bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,860,946	2,488,135
Gross Unrealized Gains	37,717	36,657
Gross Unrealized Losses	975	550
Fair Value	2,897,688	2,524,242
Taxable bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,330,983	1,226,136
Gross Unrealized Gains	19,120	20,015
Gross Unrealized Losses	492	1,514
Fair Value	1,349,611	1,244,637
Restricted investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	170,000	170,000
Gross Unrealized Losses	776	3,019
Fair Value	169,224	166,981
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	362,208	296,105
Gross Unrealized Gains	44,044	35,564
Gross Unrealized Losses	16,721	14,274
Fair Value	$ 389,531	$ 317,395

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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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.

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- Definition

This item represents the gross unrealized gains for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities.

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