Company's Cash Equivalents and Investments Measured at Fair Value (Detail) (Fair Value, Measurements, Recurring [Member], USD $) In Thousands, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Company's Cash Equivalents and Investments Measured at Fair Value [Line Items]
Cash equivalents measured at fair value	$ 339,690
Investments measured at fair value	202,520		155,644
Fair Value Measurements Level 1 [Member]
Company's Cash Equivalents and Investments Measured at Fair Value [Line Items]
Cash equivalents measured at fair value	338,690
Investments measured at fair value	13,276		5,403
Fair Value Measurements Level 2 [Member]
Company's Cash Equivalents and Investments Measured at Fair Value [Line Items]
Cash equivalents measured at fair value	1,000
Investments measured at fair value	189,244		150,241
Money-market funds [Member]
Company's Cash Equivalents and Investments Measured at Fair Value [Line Items]
Cash equivalents measured at fair value	338,690		674,219
Money-market funds [Member] | Fair Value Measurements Level 1 [Member]
Company's Cash Equivalents and Investments Measured at Fair Value [Line Items]
Cash equivalents measured at fair value	338,690	[1]	674,219	[1]
Money-market funds [Member] | Fair Value Measurements Level 2 [Member]
Company's Cash Equivalents and Investments Measured at Fair Value [Line Items]
Cash equivalents measured at fair value	0		0
Agency securities [Member]
Company's Cash Equivalents and Investments Measured at Fair Value [Line Items]
Investments measured at fair value	124,247		97,408
Agency securities [Member] | Fair Value Measurements Level 1 [Member]
Company's Cash Equivalents and Investments Measured at Fair Value [Line Items]
Investments measured at fair value	0		0
Agency securities [Member] | Fair Value Measurements Level 2 [Member]
Company's Cash Equivalents and Investments Measured at Fair Value [Line Items]
Investments measured at fair value	124,247	[2]	97,408	[2]
Non-agency securities [Member]
Company's Cash Equivalents and Investments Measured at Fair Value [Line Items]
Investments measured at fair value	2,336		9,989
Non-agency securities [Member] | Fair Value Measurements Level 1 [Member]
Company's Cash Equivalents and Investments Measured at Fair Value [Line Items]
Investments measured at fair value	0		0
Non-agency securities [Member] | Fair Value Measurements Level 2 [Member]
Company's Cash Equivalents and Investments Measured at Fair Value [Line Items]
Investments measured at fair value	2,336	[2]	9,989	[2]
U.S. government and agency securities [Member]
Company's Cash Equivalents and Investments Measured at Fair Value [Line Items]
Cash equivalents measured at fair value	1,000
Investments measured at fair value	59,244		35,975
U.S. government and agency securities [Member] | Fair Value Measurements Level 1 [Member]
Company's Cash Equivalents and Investments Measured at Fair Value [Line Items]
Cash equivalents measured at fair value	0
Investments measured at fair value	13,276	[1]	5,403	[1]
U.S. government and agency securities [Member] | Fair Value Measurements Level 2 [Member]
Company's Cash Equivalents and Investments Measured at Fair Value [Line Items]
Cash equivalents measured at fair value	1,000	[2]
Investments measured at fair value	45,968	[2]	30,572	[2]
Corporate debt securities [Member]
Company's Cash Equivalents and Investments Measured at Fair Value [Line Items]
Investments measured at fair value	16,693		12,272
Corporate debt securities [Member] | Fair Value Measurements Level 1 [Member]
Company's Cash Equivalents and Investments Measured at Fair Value [Line Items]
Investments measured at fair value	0		0
Corporate debt securities [Member] | Fair Value Measurements Level 2 [Member]
Company's Cash Equivalents and Investments Measured at Fair Value [Line Items]
Investments measured at fair value	16,693	[2]	12,272	[2]
Long-term marketable equity securities [Member]
Company's Cash Equivalents and Investments Measured at Fair Value [Line Items]
Investments measured at fair value	2,209		1,514
Long-term marketable equity securities [Member] | Fair Value Measurements Level 1 [Member]
Company's Cash Equivalents and Investments Measured at Fair Value [Line Items]
Investments measured at fair value	2,209	[3]	1,514	[3]
Long-term marketable equity securities [Member] | Fair Value Measurements Level 2 [Member]
Company's Cash Equivalents and Investments Measured at Fair Value [Line Items]
Investments measured at fair value	$ 0		$ 0

[1] The fair value of money-market funds is determined using unadjusted prices in active markets. The fair value of Level 1 U.S. government and agency securities is determined using quoted prices in active markets.

[2] These investments are traded less frequently than Level 1 securities and are valued using inputs that include quoted prices for similar assets in active markets and inputs other than quoted prices that are observable for the asset, such as interest rates, yield curves, prepayment speeds, collateral performance, broker/dealer quotes and indices that are observable at commonly quoted intervals.

[3] The fair value of marketable equity securities is determined using quoted market prices in active markets. These amounts are included within other assets in the condensed consolidated balance sheets.