Quarterly report pursuant to Section 13 or 15(d)

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

v3.8.0.1
Investments in Marketable Securities - Schedule of Amortized Cost and Fair Value of Marketable Securities, Available-for-Sale, by Contractual Maturity (Detail) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]    
Due in one year or less, Amortized Cost $ 95,045 $ 73,659
Due after one year through five years, Amortized Cost 14,733 30,644
Due after five years through ten years, Amortized Cost 14,956 15,090
Due after ten years, Amortized Cost 6,418 6,368
Amortized Cost 131,152 125,761
Due in one year or less, Fair Value 94,826 73,560
Due after one year through five years, Fair Value 14,608 30,517
Due after five years through ten years, Fair Value 14,645 15,200
Due after ten years, Fair Value 6,320 6,382
Total Fair Value $ 130,399 $ 125,659
Weighted average contractual maturity 2 years 6 months 2 years 7 months 6 days