Fair Value Measurements - Investments Classified as Available-for-Sale (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Fair Value Disclosures [Abstract]
Available-for-sale Debt Securities, Amortized cost	$ 251,673
Available-for-sale Debt Securities, Gross unrealized gains	16
Available-for-sale Debt Securities, Gross unrealized losses	(69)
Available-for-sale debt securities	251,620
Available-for-sale Equity Securities, Amortized cost	4,312
Available-for-sale Equity Securities, Gross unrealized gains	0
Available-for-sale Equity Securities, Gross unrealized losses	(1,078)
Available-for-sale equity securities	3,234
Available-for-sale Investments, Amortized cost	255,985	203,479
Available-for-sale Investments, Gross unrealized gains	16	24
Available-for-sale Investments, Gross unrealized losses	(1,147)	(23)
Available-for-sale Investments	$ 254,854us-gaap_AvailableForSaleSecurities	$ 203,480us-gaap_AvailableForSaleSecurities

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

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

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Amount of debt securities categorized neither as held-to-maturity nor trading.

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