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	Sep. 30, 2012	Dec. 31, 2011
Gain (Loss) on Investments [Line Items]
Less Than 12 Months Fair Value	$ 666,531
Less Than 12 Months Gross Unrealized Losses	(2,879)
12 Months or Greater Fair Value	90,234
12 Months or Greater Gross Unrealized Losses	(6,348)
Total Fair Value	756,765
Total Gross Unrealized Losses	(9,227)
US Government Agencies Debt Securities
Gain (Loss) on Investments [Line Items]
Less Than 12 Months Fair Value	247,834
Less Than 12 Months Gross Unrealized Losses	(906)
12 Months or Greater Fair Value	9,186
12 Months or Greater Gross Unrealized Losses	(99)
Total Fair Value	257,020
Total Gross Unrealized Losses	(1,005)	(1,488)
US Corporate Debt Securities
Gain (Loss) on Investments [Line Items]
Less Than 12 Months Fair Value	96,205
Less Than 12 Months Gross Unrealized Losses	(91)
12 Months or Greater Fair Value	2,234
12 Months or Greater Gross Unrealized Losses	(6)
Total Fair Value	98,439
Total Gross Unrealized Losses	(97)	(2,573)
High Yield Corporate Debt Securities
Gain (Loss) on Investments [Line Items]
Less Than 12 Months Fair Value	30,208
Less Than 12 Months Gross Unrealized Losses	(697)
12 Months or Greater Fair Value	1,354
12 Months or Greater Gross Unrealized Losses	(379)
Total Fair Value	31,562
Total Gross Unrealized Losses	(1,076)	(7,742)
Municipal Obligations
Gain (Loss) on Investments [Line Items]
Less Than 12 Months Fair Value	192,338
Less Than 12 Months Gross Unrealized Losses	(362)
12 Months or Greater Fair Value	2,614
12 Months or Greater Gross Unrealized Losses	(4)
Total Fair Value	194,952
Total Gross Unrealized Losses	(366)	(597)
Auction Rate Securities
Gain (Loss) on Investments [Line Items]
12 Months or Greater Fair Value	74,846
12 Months or Greater Gross Unrealized Losses	(5,860)
Total Fair Value	74,846
Total Gross Unrealized Losses	(5,860)	(8,304)
Foreign Debt Securities
Gain (Loss) on Investments [Line Items]
Less Than 12 Months Fair Value	98,849
Less Than 12 Months Gross Unrealized Losses	(198)
Total Fair Value	98,849
Total Gross Unrealized Losses	(198)	(2,807)
Public Equity Securities
Gain (Loss) on Investments [Line Items]
Less Than 12 Months Fair Value	1,097
Less Than 12 Months Gross Unrealized Losses	(625)
Total Fair Value	1,097
Total Gross Unrealized Losses	$ (625)