Cash Equivalents and Investments - Company's Cash Equivalents and Investments Measured at Fair Value (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Company's Cash Equivalents and Investments measured at Fair Value [Line Items]
Cash equivalents measured at fair value	$ 273,844	$ 371,075
Investments measured at fair value	263,908	204,457
Money-market funds [Member]
Company's Cash Equivalents and Investments measured at Fair Value [Line Items]
Cash equivalents measured at fair value	268,320	364,596
Government and agency securities [Member]
Company's Cash Equivalents and Investments measured at Fair Value [Line Items]
Cash equivalents measured at fair value	600	6,479
Investments measured at fair value	84,641	66,700
Agency securities [Member]
Company's Cash Equivalents and Investments measured at Fair Value [Line Items]
Investments measured at fair value	144,220	129,463
Non-agency securities [Member]
Company's Cash Equivalents and Investments measured at Fair Value [Line Items]
Investments measured at fair value	845	1,393
Corporate debt securities [Member]
Company's Cash Equivalents and Investments measured at Fair Value [Line Items]
Investments measured at fair value	20,857	6,001
Commercial paper [Member]
Company's Cash Equivalents and Investments measured at Fair Value [Line Items]
Cash equivalents measured at fair value	4,924
Investments measured at fair value	13,345	900
Long-term investments in equity securities [Member]
Company's Cash Equivalents and Investments measured at Fair Value [Line Items]
Investments measured at fair value	1,990	1,689
Level 1 [Member]
Company's Cash Equivalents and Investments measured at Fair Value [Line Items]
Cash equivalents measured at fair value	268,320	364,596
Investments measured at fair value	23,441	17,042
Level 1 [Member] | Money-market funds [Member]
Company's Cash Equivalents and Investments measured at Fair Value [Line Items]
Cash equivalents measured at fair value	268,320	364,596
Level 1 [Member] | Government and agency securities [Member]
Company's Cash Equivalents and Investments measured at Fair Value [Line Items]
Investments measured at fair value	23,441	17,042
Level 1 [Member] | Long-term investments in equity securities [Member]
Company's Cash Equivalents and Investments measured at Fair Value [Line Items]
Investments measured at fair value	1,990	1,689
Level 2 [Member]
Company's Cash Equivalents and Investments measured at Fair Value [Line Items]
Cash equivalents measured at fair value	5,524	6,479
Investments measured at fair value	240,467	187,415
Level 2 [Member] | Government and agency securities [Member]
Company's Cash Equivalents and Investments measured at Fair Value [Line Items]
Cash equivalents measured at fair value	600	6,479
Investments measured at fair value	61,200	49,658
Level 2 [Member] | Agency securities [Member]
Company's Cash Equivalents and Investments measured at Fair Value [Line Items]
Investments measured at fair value	144,220	129,463
Level 2 [Member] | Non-agency securities [Member]
Company's Cash Equivalents and Investments measured at Fair Value [Line Items]
Investments measured at fair value	845	1,393
Level 2 [Member] | Corporate debt securities [Member]
Company's Cash Equivalents and Investments measured at Fair Value [Line Items]
Investments measured at fair value	20,857	6,001
Level 2 [Member] | Commercial paper [Member]
Company's Cash Equivalents and Investments measured at Fair Value [Line Items]
Cash equivalents measured at fair value	4,924
Investments measured at fair value	$ 13,345	$ 900