Investments - Gains Unrealized Gains and (Losses) Aggregated by Investment Category Short-Term and Long-Term Securities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Short-term investments, Cost	$ 330,646	$ 326,601
Short-term investments, Gross Unrealized Gains	177	267
Short-term investments, Gross Unrealized (Losses)	(2)	(6)
Short-term investments, Estimated Fair Value	330,821	326,862
Long-term investments, Cost	10,152
Long-term investments, Gross Unrealized Gains	5
Long-term investments, Gross Unrealized (Losses)
Long-term investments, Estimated Fair Value	10,157	12,158
U.S. agency obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Short-term investments, Cost	317,517	313,262
Short-term investments, Gross Unrealized Gains	171	258
Short-term investments, Gross Unrealized (Losses)	(2)	(6)
Short-term investments, Estimated Fair Value	317,686	313,514
Long-term investments, Cost	10,099	12,156
Long-term investments, Gross Unrealized Gains	5	2
Long-term investments, Gross Unrealized (Losses)
Long-term investments, Estimated Fair Value	10,104	12,158
Time deposits [Member]
Schedule of Available-for-sale Securities [Line Items]
Short-term investments, Cost		52
Short-term investments, Gross Unrealized Gains
Short-term investments, Gross Unrealized (Losses)
Short-term investments, Estimated Fair Value		52
Long-term investments, Cost	53
Long-term investments, Gross Unrealized Gains
Long-term investments, Gross Unrealized (Losses)
Long-term investments, Estimated Fair Value	53
Bankers' acceptance drafts [Member]
Schedule of Available-for-sale Securities [Line Items]
Short-term investments, Cost	93	242
Short-term investments, Gross Unrealized Gains
Short-term investments, Gross Unrealized (Losses)
Short-term investments, Estimated Fair Value	93	242
U.S. treasury obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Short-term investments, Cost	13,036	13,045
Short-term investments, Gross Unrealized Gains	6	9
Short-term investments, Gross Unrealized (Losses)
Short-term investments, Estimated Fair Value	$ 13,042	$ 13,054