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	Mar. 31, 2013
Gain (Loss) on Investments [Line Items]
Less Than 12 Months Fair Value	$ 898,766
Less than 12 Months Unrealized Losses	(3,717)
12 Months or Greater Fair Value	87,299
12 Months or Greater Unrealized Losses	(4,150)
Total Fair Value	986,065
Total, Gross Unrealized Losses	7,867
US Government Agencies Debt Securities
Gain (Loss) on Investments [Line Items]
Less Than 12 Months Fair Value	254,486
Less than 12 Months Unrealized Losses	(1,235)
12 Months or Greater Fair Value	9,295
12 Months or Greater Unrealized Losses	(216)
Total Fair Value	263,781
Total, Gross Unrealized Losses	1,451
US Corporate Debt Securities
Gain (Loss) on Investments [Line Items]
Less Than 12 Months Fair Value	273,100
Less than 12 Months Unrealized Losses	(436)
12 Months or Greater Fair Value	0
12 Months or Greater Unrealized Losses	0
Total Fair Value	273,100
Total, Gross Unrealized Losses	436
High Yield Corporate Debt Securities
Gain (Loss) on Investments [Line Items]
Less Than 12 Months Fair Value	43,943
Less than 12 Months Unrealized Losses	(673)
12 Months or Greater Fair Value	164
12 Months or Greater Unrealized Losses	(93)
Total Fair Value	44,107
Total, Gross Unrealized Losses	766
Municipal Obligations
Gain (Loss) on Investments [Line Items]
Less Than 12 Months Fair Value	24,624
Less than 12 Months Unrealized Losses	(33)
12 Months or Greater Fair Value	9,587
12 Months or Greater Unrealized Losses	(12)
Total Fair Value	34,211
Total, Gross Unrealized Losses	45
Auction Rate Securities
Gain (Loss) on Investments [Line Items]
Less Than 12 Months Fair Value	0
Less than 12 Months Unrealized Losses	0
12 Months or Greater Fair Value	68,247
12 Months or Greater Unrealized Losses	(3,829)
Total Fair Value	68,247
Total, Gross Unrealized Losses	3,829
Foreign Debt Securities
Gain (Loss) on Investments [Line Items]
Less Than 12 Months Fair Value	298,632
Less than 12 Months Unrealized Losses	(720)
12 Months or Greater Fair Value	0
12 Months or Greater Unrealized Losses	0
Total Fair Value	298,632
Total, Gross Unrealized Losses	720
Publicly Traded Equity Securities
Gain (Loss) on Investments [Line Items]
Less Than 12 Months Fair Value	1,315
Less than 12 Months Unrealized Losses	(616)
12 Months or Greater Fair Value	0
12 Months or Greater Unrealized Losses	0
Total Fair Value	1,315
Total, Gross Unrealized Losses	$ 616