Amortized Cost and Fair Value of Marketable Securities Classified by Stated Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 183,415	$ 117,861
Fair value	183,671	118,187
Due in 1 year or less
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	100,261	53,761
Fair value	100,379	53,852
Due in 1 - 2 years
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	47,695	36,510
Fair value	47,795	36,694
Due in 2 - 3 years
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	13,397	11,847
Fair value	13,387	11,856
Due in 3 - 4 years
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	4,546	744
Fair value	4,544	746
Due in 4 - 9 years
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	5,653	5,158
Fair value	5,680	5,179
Due in 9 - 20 years
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	2,262	1,032
Fair value	2,269	1,040
Due in 20 - 33 years
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	9,601	8,809
Fair value	$ 9,617	$ 8,820

Amount of available-for-sale debt securities at cost, net of adjustments, which include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings (less any cumulative-effect adjustments, as defined) and fair value hedge accounting adjustments.

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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 represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

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