Debt and Equity Securities (Investments in Debt Securities, Classified as Available-For-Sale) (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 4,262,095	$ 3,432,466
Gross unrealized, gains	35,749	34,076
Gross unrealized, losses	(23,728)	(16,290)
Debt securities, Estimated fair value	4,274,116	3,450,252
U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	112,624	64,195
Gross unrealized, gains	803	968
Gross unrealized, losses	(622)	(181)
Debt securities, Estimated fair value	112,805	64,982
Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	682,228	577,703
Gross unrealized, gains	12,136	10,981
Gross unrealized, losses	(1,376)	(1,007)
Debt securities, Estimated fair value	692,988	587,677
Foreign Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	179,546	194,749
Gross unrealized, gains	2,234	2,009
Gross unrealized, losses	(470)	(8)
Debt securities, Estimated fair value	181,310	196,750
Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	420,178	198,330
Gross unrealized, gains	1,999	1,562
Gross unrealized, losses	(840)	(2,018)
Debt securities, Estimated fair value	421,337	197,874
Governmental Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	2,071,934	1,812,766
Gross unrealized, gains	11,367	8,491
Gross unrealized, losses	(5,884)	(9,095)
Debt securities, Estimated fair value	2,077,417	1,812,162
Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	795,585	568,774
Gross unrealized, gains	7,210	8,759
Gross unrealized, losses	(14,536)	(3,264)
Debt securities, Estimated fair value	$ 788,259	574,269
Non-Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost		15,949
Gross unrealized, gains		1,306
Gross unrealized, losses		(717)
Debt securities, Estimated fair value		$ 16,538

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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This item represents the cost of debt 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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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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

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