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Financial Instruments - Summary of Bond Activity (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Derivative Instruments And Hedging Activities Disclosure [Abstract]    
Fair value at beginning of period $ 51,101 $ 44,573
Purchases 18,426 15,546
Sales (1,793) (940)
Maturities and calls (10,445) (7,845)
Decrease in bond premium (377) (160)
Fair market value adjustment (356) (73)
Fair value at end of period $ 56,556 $ 51,101