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Long-Term Investments - Rollforward of Credit (Gain) Loss Recognized in Earnings on Failed Auction Security (Detail) - Failed Auction Security [Member] - USD ($)
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]    
Balance at the beginning of the period $ 59,000 $ 72,000
Reductions in the amount related to credit gain for which other-than-temporary impairment was not previously recognized (3,000) (4,000)
Balance at the end of the period $ 56,000 $ 68,000