Investments and Fair Value Disclosures - Summary of Long-Term Investments (Detail) (USD $) In Thousands, unless otherwise specified			3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Money market funds [Member]	Mar. 31, 2013 U.S. agency notes [Member]	Mar. 31, 2013 Commercial paper [Member]	Mar. 31, 2013 Corporate bonds [Member]
Schedule Of Investments In Marketable Securities [Line Items]
Contracted Maturity of cash and cash equivalents			demand
Contracted Maturity of short-term investments				76 - 277 days	2 - 306 days	5 - 299 days
Contracted Maturity of long-term investments				472 - 717 days		408 - 593 days
Total cash equivalents, Carrying Value	$ 54,026		$ 54,026
Total short-term investments, Carrying Value	74,893	73,075		21,526	28,978	24,389
Total long-term investments, Carrying Value	$ 37,435	$ 30,102		$ 26,617		$ 10,818