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Financial Instruments - Summary of Bond Activity (Detail) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Derivative Instruments And Hedging Activities Disclosure [Abstract]    
Fair value at beginning of period $ 51,101 $ 44,573
Purchases 6,170 15,546
Redemptions   (940)
Maturities and calls (3,345) (7,845)
Change in bond premium 130 (160)
Fair market value adjustment (454) (73)
Fair value at end of period $ 53,602 $ 51,101