Quarterly report pursuant to Section 13 or 15(d)

Impact of Derivative Activities on Condensed Consolidated Statements of Income (Detail)

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Impact of Derivative Activities on Condensed Consolidated Statements of Income (Detail) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]        
Derivatives $ (40,689) $ (12,275) $ (111,725) $ 15,802