Investments - Components of Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	$ 235,149	$ 203,362
Unrealized Gains	18	21
Unrealized Losses	(282)	(268)
Fair Value	234,885	203,115
Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	232,671	200,976
Unrealized Gains	18	21
Unrealized Losses	(282)	(268)
Fair Value	232,407	200,729
Investments [Member] \| Money Market Mutual Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	35,407	68,612
Fair Value	35,407	68,612
Investments [Member] \| Municipal Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis		1,500
Unrealized Gains		2
Fair Value		1,502
Investments [Member] \| Corporate Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	185,761	130,864
Unrealized Gains	10	19
Unrealized Losses	(278)	(268)
Fair Value	185,493	130,615
Investments [Member] \| US Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	11,503
Unrealized Gains	8
Unrealized Losses	(4)
Fair Value	11,507
Deferred Compensation Plan Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	2,478	2,386
Fair Value	$ 2,478	$ 2,386

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

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