Cash and Cash Equivalents, Short-Term Investments and Restricted Investments (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Cash Cash Equivalents Short Term Investments and Restricted Investments [Line Items]
Cost	$ 117,078	$ 140,935
Gross Unrealized Gains	20	33
Gross Unrealized Losses	(17)	(11)
Estimated Fair Value	117,081	140,957
US Treasury Securities
Cash Cash Equivalents Short Term Investments and Restricted Investments [Line Items]
Cost	4,000
Estimated Fair Value	4,000
U.S. government-sponsored agencies
Cash Cash Equivalents Short Term Investments and Restricted Investments [Line Items]
Cost	4,397	11,329
Gross Unrealized Gains	1	1
Estimated Fair Value	4,398	11,330
Corporate debt securities
Cash Cash Equivalents Short Term Investments and Restricted Investments [Line Items]
Cost	95,171	105,839
Gross Unrealized Gains	19	32
Gross Unrealized Losses	(17)	(11)
Estimated Fair Value	95,173	105,860
Certificates of deposit
Cash Cash Equivalents Short Term Investments and Restricted Investments [Line Items]
Cost	1,955	1,955
Estimated Fair Value	1,955	1,955
Short-term investments
Cash Cash Equivalents Short Term Investments and Restricted Investments [Line Items]
Estimated Fair Value	94,115	102,868
Restricted investments
Cash Cash Equivalents Short Term Investments and Restricted Investments [Line Items]
Estimated Fair Value	1,955	1,955
Cash
Cash Cash Equivalents Short Term Investments and Restricted Investments [Line Items]
Cost	1,943	915
Estimated Fair Value	1,943	915
Money market funds
Cash Cash Equivalents Short Term Investments and Restricted Investments [Line Items]
Cost	9,612	20,897
Estimated Fair Value	9,612	20,897
Cash and cash equivalents
Cash Cash Equivalents Short Term Investments and Restricted Investments [Line Items]
Estimated Fair Value	$ 21,011	$ 36,134