Fair Value Measurements (Detail) - Financial Assets Measured At Fair Value On A Recurring Basis (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Cash equivalents
Cash equivalents	$ 68,752	$ 47,462
Short-term investments
Short-term investments	160,027	126,092
Municipal bonds [Member]
Short-term investments
Short-term investments	150,489	105,515
U.S. government and agency securities [Member]
Short-term investments
Short-term investments	5,499
Corporate bonds [Member]
Short-term investments
Short-term investments	4,039	6,880
Commercial paper [Member]
Short-term investments
Short-term investments		13,697
Commercial paper [Member]
Cash equivalents
Cash equivalents	62,696	19,800
Money market funds [Member]
Cash equivalents
Cash equivalents	5,017	14,670
Muncipal bonds [Member]
Cash equivalents
Cash equivalents	1,039	3,992
U.S. government and agency securities [Member]
Cash equivalents
Cash equivalents		9,000
Fair Value, Inputs, Level 1 [Member]
Cash equivalents
Cash equivalents	5,017	14,670
Fair Value, Inputs, Level 1 [Member] \| Money market funds [Member]
Cash equivalents
Cash equivalents	5,017	14,670
Fair Value, Inputs, Level 2 [Member]
Cash equivalents
Cash equivalents	63,735	32,792
Short-term investments
Short-term investments	160,027	126,092
Fair Value, Inputs, Level 2 [Member] \| Municipal bonds [Member]
Short-term investments
Short-term investments	150,489	105,515
Fair Value, Inputs, Level 2 [Member] \| U.S. government and agency securities [Member]
Short-term investments
Short-term investments	5,499
Fair Value, Inputs, Level 2 [Member] \| Corporate bonds [Member]
Short-term investments
Short-term investments	4,039	6,880
Fair Value, Inputs, Level 2 [Member] \| Commercial paper [Member]
Short-term investments
Short-term investments		13,697
Fair Value, Inputs, Level 2 [Member] \| Commercial paper [Member]
Cash equivalents
Cash equivalents	62,696	19,800
Fair Value, Inputs, Level 2 [Member] \| Muncipal bonds [Member]
Cash equivalents
Cash equivalents	1,039	3,992
Fair Value, Inputs, Level 2 [Member] \| U.S. government and agency securities [Member]
Cash equivalents
Cash equivalents		$ 9,000

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the Company may deposit additional funds at any time and also effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months.

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