Composition of Investments (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Unrealized Losses	$ (13,574)
US Government Agencies Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Unrealized Losses	(1,231)
US Corporate Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Unrealized Losses	(642)
High Yield Corporate Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Unrealized Losses	(2,844)
Asset-backed Securities
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Unrealized Losses	(11)
Municipal Obligations
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Unrealized Losses	(852)
Auction Rate Securities
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Unrealized Losses	(6,810)
Foreign Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Unrealized Losses	(551)
Publicly Traded Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Unrealized Losses	(633)
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Amortized Cost	6,771,591	6,228,479
Available For Sale Securities Unrealized Gains	50,655	42,149
Available For Sale Securities Unrealized Losses	(12,941)	(23,536)
Available For Sale Securities Fair Value	6,809,305	6,247,092
Debt Securities \| US Government Agencies Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Amortized Cost	2,301,054	2,474,029
Available For Sale Securities Unrealized Gains	9,614	12,420
Available For Sale Securities Unrealized Losses	(1,231)	(1,488)
Available For Sale Securities Fair Value	2,309,437	2,484,961
Debt Securities \| US Corporate Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Amortized Cost	1,498,162	1,400,373
Available For Sale Securities Unrealized Gains	9,087	9,953
Available For Sale Securities Unrealized Losses	(642)	(2,573)
Available For Sale Securities Fair Value	1,506,607	1,407,753
Debt Securities \| High Yield Corporate Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Amortized Cost	442,573	442,723
Available For Sale Securities Unrealized Gains	22,658	12,498
Available For Sale Securities Unrealized Losses	(2,844)	(7,742)
Available For Sale Securities Fair Value	462,387	447,479
Debt Securities \| Asset-backed Securities
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Amortized Cost	39,898	29,101
Available For Sale Securities Unrealized Gains	85	72
Available For Sale Securities Unrealized Losses	(11)	(25)
Available For Sale Securities Fair Value	39,972	29,148
Debt Securities \| Municipal Obligations
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Amortized Cost	1,217,070	814,657
Available For Sale Securities Unrealized Gains	2,860	2,021
Available For Sale Securities Unrealized Losses	(852)	(597)
Available For Sale Securities Fair Value	1,219,078	816,081
Debt Securities \| Auction Rate Securities
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Amortized Cost	80,706	82,900
Available For Sale Securities Unrealized Losses	(6,810)	(8,304)
Available For Sale Securities Fair Value	73,896	74,596
Debt Securities \| Foreign Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Amortized Cost	1,192,128	984,696
Available For Sale Securities Unrealized Gains	6,351	5,185
Available For Sale Securities Unrealized Losses	(551)	(2,807)
Available For Sale Securities Fair Value	1,197,928	987,074
Debt and Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Amortized Cost	6,818,024	6,286,678
Available For Sale Securities Unrealized Gains	108,610	48,951
Available For Sale Securities Unrealized Losses	(13,574)	(23,536)
Available For Sale Securities Fair Value	6,913,060	6,312,093
Debt and Equity Securities \| Publicly Traded Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Amortized Cost	46,433	58,199
Available For Sale Securities Unrealized Gains	57,955	6,802
Available For Sale Securities Unrealized Losses	(633)
Available For Sale Securities Fair Value	$ 103,755	$ 65,001

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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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

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 22

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

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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

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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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-SubTopic 10

-Section 25

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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