Short-term Investments (Detail) - Summary Of Investments In Available-For-Sale Securities (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Investment Holdings [Line Items]
Amortized Cost	$ 239,621	$ 190,519
Unrealized Gains	25	58
Unrealized Losses	(132)	(10)
Estimated Fair Value	239,514us-gaap_AvailableForSaleSecurities	190,567us-gaap_AvailableForSaleSecurities
Municipal bonds
Investment Holdings [Line Items]
Amortized Cost	120,270	146,047
Unrealized Gains	24	56
Unrealized Losses	(12)	(3)
Estimated Fair Value	120,282	146,100
Commercial paper
Investment Holdings [Line Items]
Amortized Cost	2,498	31,597
Unrealized Gains	0	0
Unrealized Losses	0	0
Estimated Fair Value	2,498	31,597
Corporate bonds
Investment Holdings [Line Items]
Amortized Cost	68,758	7,873
Unrealized Gains	1	2
Unrealized Losses	(87)	0
Estimated Fair Value	68,672	7,875
U.S. government and agency securities
Investment Holdings [Line Items]
Amortized Cost	48,095	5,002
Unrealized Gains	0	0
Unrealized Losses	(33)	(7)
Estimated Fair Value	$ 48,062	$ 4,995

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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