Composition of Investments (Detail) (USD $)	Mar. 31, 2012	Dec. 31, 2011
Debt Securities
Available For Sale Securities Amortized Cost	$ 6,084,240,000	$ 6,228,479,000
Available For Sale Securities Unrealized Gains	52,179,000	42,149,000
Available For Sale Securities Unrealized Losses	(10,620,000)	(23,536,000)
Available For Sale Securities Fair Value	6,125,799,000	6,247,092,000
Debt Securities \| US Government Agencies Debt Securities
Available For Sale Securities Amortized Cost	2,212,101,000	2,474,029,000
Available For Sale Securities Unrealized Gains	10,111,000	12,420,000
Available For Sale Securities Unrealized Losses	(1,680,000)	(1,488,000)
Available For Sale Securities Fair Value	2,220,532,000	2,484,961,000
Debt Securities \| US Corporate Debt Securities
Available For Sale Securities Amortized Cost	1,371,048,000	1,400,373,000
Available For Sale Securities Unrealized Gains	11,498,000	9,953,000
Available For Sale Securities Unrealized Losses	(475,000)	(2,573,000)
Available For Sale Securities Fair Value	1,382,071,000	1,407,753,000
Debt Securities \| High Yield Corporate Debt Securities
Available For Sale Securities Amortized Cost	451,107,000	442,723,000
Available For Sale Securities Unrealized Gains	21,441,000	12,498,000
Available For Sale Securities Unrealized Losses	(3,199,000)	(7,742,000)
Available For Sale Securities Fair Value	469,349,000	447,479,000
Debt Securities \| Asset-backed Securities
Available For Sale Securities Amortized Cost	42,766,000	29,101,000
Available For Sale Securities Unrealized Gains	105,000	72,000
Available For Sale Securities Unrealized Losses	(15,000)	(25,000)
Available For Sale Securities Fair Value	42,856,000	29,148,000
Debt Securities \| Municipal Obligations
Available For Sale Securities Amortized Cost	898,319,000	814,657,000
Available For Sale Securities Unrealized Gains	2,316,000	2,021,000
Available For Sale Securities Unrealized Losses	(655,000)	(597,000)
Available For Sale Securities Fair Value	899,980,000	816,081,000
Debt Securities \| Auction Rate Securities
Available For Sale Securities Amortized Cost	82,675,000	82,900,000
Available For Sale Securities Unrealized Gains	0	0
Available For Sale Securities Unrealized Losses	(4,278,000)	(8,304,000)
Available For Sale Securities Fair Value	78,397,000	74,596,000
Debt Securities \| Foreign Debt Securities
Available For Sale Securities Amortized Cost	1,026,224,000	984,696,000
Available For Sale Securities Unrealized Gains	6,708,000	5,185,000
Available For Sale Securities Unrealized Losses	(318,000)	(2,807,000)
Available For Sale Securities Fair Value	1,032,614,000	987,074,000
Debt and Equity Securities
Available For Sale Securities Amortized Cost	6,142,299,000	6,286,678,000
Available For Sale Securities Unrealized Gains	75,689,000	48,951,000
Available For Sale Securities Unrealized Losses	(10,620,000)	(23,536,000)
Available For Sale Securities Fair Value	6,207,368,000	6,312,093,000
Debt and Equity Securities \| Publicly Traded Equity Securities
Available For Sale Securities Amortized Cost	58,059,000	58,199,000
Available For Sale Securities Unrealized Gains	23,510,000	6,802,000
Available For Sale Securities Unrealized Losses	0	0
Available For Sale Securities Fair Value	$ 81,569,000	$ 65,001,000

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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-Section 45

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

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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)

-Number FAS115-1/124-1

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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 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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-SubTopic 10

-Section 25

-Paragraph 1

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-URI http://asc.fasb.org/extlink&oid=6379932&loc=d3e22054-111558

+ 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