Annual report pursuant to Section 13 and 15(d)

Investments in Marketable Securities - Schedule of Amortized Cost and Fair Value of Marketable Securities, Available-for-Sale, by Contractual Maturity (Detail)

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Investments in Marketable Securities - Schedule of Amortized Cost and Fair Value of Marketable Securities, Available-for-Sale, by Contractual Maturity (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]    
Due in one year or less, Amortized Cost $ 73,659 $ 27,484
Due after one year through five years, Amortized Cost 30,644 57,309
Due after five years through ten years, Amortized Cost 15,090 14,992
Due after ten years, Amortized Cost 6,368 5,580
Amortized Cost 125,761 105,365
Due in one year or less, Fair Value 73,560 27,454
Due after one year through five years, Fair Value 30,517 57,144
Due after five years through ten years, Fair Value 15,200 14,841
Due after ten years, Fair Value 6,382 5,490
Total Fair Value $ 125,659 $ 104,929
Weighted average maturity 2 years 7 months 6 days 3 years 6 months