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Financial Instruments - Summary of Bond Activity (Detail) - USD ($)
$ in Thousands
 6 Months Ended 12 Months Ended
Jun. 30, 2016
Dec. 31, 2015
Derivative Instruments And Hedging Activities Disclosure [Abstract]    
Fair value at beginning of period $ 43,142 $ 38,399
Purchases 4,963 15,429
Redemptions (1,490) (987)
Maturities (3,257) (9,677)
Increase in bond premium (205) 172
Fair market value adjustment 572 (194)
Fair value at end of period $ 43,725 $ 43,142