Composition of Investments (Detail) (USD $)	Dec. 31, 2011	Dec. 31, 2010
Debt Securities
Schedule of Investments [Line Items]
Available For Sale Securities Amortized Cost	$ 6,228,479,000	$ 5,337,845,000
Available For Sale Securities Unrealized Gains	42,149,000	54,059,000
Available For Sale Securities Unrealized Losses	(23,536,000)	(19,811,000)
Available For Sale Securities Fair Value	6,247,092,000	5,372,093,000
Debt Securities \| US Government Agencies Debt Securities
Schedule of Investments [Line Items]
Available For Sale Securities Amortized Cost	2,474,029,000	1,737,782,000
Available For Sale Securities Unrealized Gains	12,420,000	11,286,000
Available For Sale Securities Unrealized Losses	(1,488,000)	(2,674,000)
Available For Sale Securities Fair Value	2,484,961,000	1,746,394,000
Debt Securities \| US Corporate Debt Securities
Schedule of Investments [Line Items]
Available For Sale Securities Amortized Cost	1,400,373,000	1,239,325,000
Available For Sale Securities Unrealized Gains	9,953,000	13,608,000
Available For Sale Securities Unrealized Losses	(2,573,000)	(1,307,000)
Available For Sale Securities Fair Value	1,407,753,000	1,251,626,000
Debt Securities \| High Yield Corporate Debt Securities
Schedule of Investments [Line Items]
Available For Sale Securities Amortized Cost	442,723,000	421,469,000
Available For Sale Securities Unrealized Gains	12,498,000	18,306,000
Available For Sale Securities Unrealized Losses	(7,742,000)	(1,943,000)
Available For Sale Securities Fair Value	447,479,000	437,832,000
Debt Securities \| Asset-backed Securities
Schedule of Investments [Line Items]
Available For Sale Securities Amortized Cost	29,101,000	34,730,000
Available For Sale Securities Unrealized Gains	72,000	152,000
Available For Sale Securities Unrealized Losses	(25,000)	(1,000)
Available For Sale Securities Fair Value	29,148,000	34,881,000
Debt Securities \| Municipal Obligations
Schedule of Investments [Line Items]
Available For Sale Securities Amortized Cost	814,657,000	1,095,338,000
Available For Sale Securities Unrealized Gains	2,021,000	3,829,000
Available For Sale Securities Unrealized Losses	(597,000)	(3,266,000)
Available For Sale Securities Fair Value	816,081,000	1,095,901,000
Debt Securities \| Auction Rate Securities
Schedule of Investments [Line Items]
Available For Sale Securities Amortized Cost	82,900,000	155,950,000
Available For Sale Securities Unrealized Gains	0	0
Available For Sale Securities Unrealized Losses	(8,304,000)	(9,906,000)
Available For Sale Securities Fair Value	74,596,000	146,044,000
Debt Securities \| Foreign Debt Securities
Schedule of Investments [Line Items]
Available For Sale Securities Amortized Cost	984,696,000	653,251,000
Available For Sale Securities Unrealized Gains	5,185,000	6,878,000
Available For Sale Securities Unrealized Losses	(2,807,000)	(714,000)
Available For Sale Securities Fair Value	987,074,000	659,415,000
Debt and Equity Securities
Schedule of Investments [Line Items]
Available For Sale Securities Amortized Cost	6,286,678,000	5,360,221,000
Available For Sale Securities Unrealized Gains	48,951,000	86,507,000
Available For Sale Securities Unrealized Losses	(23,536,000)	(19,811,000)
Available For Sale Securities Fair Value	6,312,093,000	5,426,917,000
Debt and Equity Securities \| Publicly Traded Equity Securities
Schedule of Investments [Line Items]
Available For Sale Securities Amortized Cost	58,199,000	22,376,000
Available For Sale Securities Unrealized Gains	6,802,000	32,448,000
Available For Sale Securities Unrealized Losses	0	0
Available For Sale Securities Fair Value	$ 65,001,000	$ 54,824,000

For an unclassified balance sheet, this item represents investments in debt and equity securities which are 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

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-Name Statement of Financial Accounting Standard (FAS)

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

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-Paragraph 22

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 13

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Staff Position (FSP)

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Reference 8: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 14

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 9: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 16

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 10: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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-Section 25

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+ Details

- Definition

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.

+ References+ Details