Investments - Components of Investments (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	$ 203,362	$ 102,673
Unrealized Gains	21	67
Unrealized Losses	(268)	(30)
Fair Value	203,115us-gaap_AvailableForSaleSecurities	102,710us-gaap_AvailableForSaleSecurities
Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	200,976	100,564
Unrealized Gains	21	67
Unrealized Losses	(268)	(30)
Fair Value	200,729	100,601
Investments [Member] \| Money Market Mutual Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	68,612	18,470
Fair Value	68,612	18,470
Investments [Member] \| Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis		2,999
Fair Value		2,999
Investments [Member] \| Municipal bonds and notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	1,500	1,500
Unrealized Gains	2
Unrealized Losses		(1)
Fair Value	1,502	1,499
Investments [Member] \| Corporate bonds and notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	130,864	77,595
Unrealized Gains	19	67
Unrealized Losses	(268)	(29)
Fair Value	130,615	77,633
Deferred Compensation Plan Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	2,386	2,109
Fair Value	$ 2,386	$ 2,109

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

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-Name Accounting Standards Codification

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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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