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Financial Instruments - Summary of Bond Activity (Detail) - USD ($)
$ in Thousands
 6 Months Ended 12 Months Ended
Jun. 30, 2017
Dec. 31, 2016
Derivative Instruments And Hedging Activities Disclosure [Abstract]    
Fair value at beginning of period $ 44,573 $ 43,142
Purchases 11,788 11,355
Redemptions (940) (2,900)
Maturities (3,435) (6,878)
Increase (decrease) in bond premium 204 (106)
Fair market value adjustment 251 (40)
Fair value at end of period $ 52,441 $ 44,573