Debt And Equity Securities (Available-For-Sale Investments Measured At Fair Value On Recurring Basis) (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	$ 2,849,801	$ 2,385,911
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	2,651,881	2,201,911
Debt Securities [Member] \| U.S. Treasury Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	82,175	74,231
Debt Securities [Member] \| Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	375,822	349,123
Debt Securities [Member] \| Foreign Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	238,745	215,020
Debt Securities [Member] \| Governmental Agency Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	325,450	197,753
Debt Securities [Member] \| Governmental Agency Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	1,281,532	1,076,547
Debt Securities [Member] \| Non-Agency Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	21,846	30,634
Debt Securities [Member] \| Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	326,311	258,603
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	197,920	184,000
Equity Securities [Member] \| Preferred Stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	14,037	7,668
Equity Securities [Member] \| Common Stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	183,883	176,332
Level 1 [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	197,920	184,000
Level 1 [Member] \| Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	197,920	184,000
Level 1 [Member] \| Equity Securities [Member] \| Preferred Stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	14,037	7,668
Level 1 [Member] \| Equity Securities [Member] \| Common Stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	183,883	176,332
Level 2 [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	2,630,035	2,171,277
Level 2 [Member] \| Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	2,630,035	2,171,277
Level 2 [Member] \| Debt Securities [Member] \| U.S. Treasury Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	82,175	74,231
Level 2 [Member] \| Debt Securities [Member] \| Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	375,822	349,123
Level 2 [Member] \| Debt Securities [Member] \| Foreign Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	238,745	215,020
Level 2 [Member] \| Debt Securities [Member] \| Governmental Agency Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	325,450	197,753
Level 2 [Member] \| Debt Securities [Member] \| Governmental Agency Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	1,281,532	1,076,547
Level 2 [Member] \| Debt Securities [Member] \| Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	326,311	258,603
Level 3 [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	21,846	30,634
Level 3 [Member] \| Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	21,846	30,634
Level 3 [Member] \| Debt Securities [Member] \| Non-Agency Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	$ 21,846	$ 30,634

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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