Fair Value of Financial Assets and Liabilities Unrealized Losses on Investments by Investment Category and Length of Time in Continuous Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Gain (Loss) on Investments [Line Items]
Less Than 12 Months Fair Value	$ 1,950,728
Less Than 12 Months Gross Unrealized Losses	(6,651)
12 Months or Greater Fair Value	84,155
12 Months or Greater Gross Unrealized Losses	(6,923)
Total Fair Value	2,034,883
Total Gross Unrealized Losses	(13,574)
US Government Agencies Debt Securities
Gain (Loss) on Investments [Line Items]
Less Than 12 Months Fair Value	821,950
Less Than 12 Months Gross Unrealized Losses	(1,170)
12 Months or Greater Fair Value	8,029
12 Months or Greater Gross Unrealized Losses	(61)
Total Fair Value	829,979
Total Gross Unrealized Losses	(1,231)
US Corporate Debt Securities
Gain (Loss) on Investments [Line Items]
Less Than 12 Months Fair Value	377,330
Less Than 12 Months Gross Unrealized Losses	(642)
Total Fair Value	377,330
Total Gross Unrealized Losses	(642)
High Yield Corporate Debt Securities
Gain (Loss) on Investments [Line Items]
Less Than 12 Months Fair Value	63,023
Less Than 12 Months Gross Unrealized Losses	(2,793)
12 Months or Greater Fair Value	2,225
12 Months or Greater Gross Unrealized Losses	(51)
Total Fair Value	65,248
Total Gross Unrealized Losses	(2,844)
Asset-backed Securities
Gain (Loss) on Investments [Line Items]
Less Than 12 Months Fair Value	9,393
Less Than 12 Months Gross Unrealized Losses	(10)
12 Months or Greater Fair Value	5
12 Months or Greater Gross Unrealized Losses	(1)
Total Fair Value	9,398
Total Gross Unrealized Losses	(11)
Municipal Obligations
Gain (Loss) on Investments [Line Items]
Less Than 12 Months Fair Value	418,354
Less Than 12 Months Gross Unrealized Losses	(852)
Total Fair Value	418,354
Total Gross Unrealized Losses	(852)
Auction Rate Securities
Gain (Loss) on Investments [Line Items]
12 Months or Greater Fair Value	73,896
12 Months or Greater Gross Unrealized Losses	(6,810)
Total Fair Value	73,896
Total Gross Unrealized Losses	(6,810)
Foreign Debt Securities
Gain (Loss) on Investments [Line Items]
Less Than 12 Months Fair Value	259,385
Less Than 12 Months Gross Unrealized Losses	(551)
Total Fair Value	259,385
Total Gross Unrealized Losses	(551)
Publicly Traded Equity Securities
Gain (Loss) on Investments [Line Items]
Less Than 12 Months Fair Value	1,293
Less Than 12 Months Gross Unrealized Losses	(633)
Total Fair Value	1,293
Total Gross Unrealized Losses	(633)
Debt Securities
Gain (Loss) on Investments [Line Items]
Total Gross Unrealized Losses	(12,941)	(23,536)
Debt Securities | US Government Agencies Debt Securities
Gain (Loss) on Investments [Line Items]
Total Gross Unrealized Losses	(1,231)	(1,488)
Debt Securities | US Corporate Debt Securities
Gain (Loss) on Investments [Line Items]
Total Gross Unrealized Losses	(642)	(2,573)
Debt Securities | High Yield Corporate Debt Securities
Gain (Loss) on Investments [Line Items]
Total Gross Unrealized Losses	(2,844)	(7,742)
Debt Securities | Asset-backed Securities
Gain (Loss) on Investments [Line Items]
Total Gross Unrealized Losses	(11)	(25)
Debt Securities | Municipal Obligations
Gain (Loss) on Investments [Line Items]
Total Gross Unrealized Losses	(852)	(597)
Debt Securities | Auction Rate Securities
Gain (Loss) on Investments [Line Items]
Total Gross Unrealized Losses	(6,810)	(8,304)
Debt Securities | Foreign Debt Securities
Gain (Loss) on Investments [Line Items]
Total Gross Unrealized Losses	$ (551)	$ (2,807)