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Derivative Financial Instruments - Summary of Gain (Loss) Recognized in Income on Oil and Gas Commodity Contracts on (Detail) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Mar. 31, 2017
Mar. 31, 2016
Call options [Member] | Net Realized Investment Gains [Member]        
Derivative Instruments, Gain (Loss) [Line Items]        
Derivative, Gain (Loss) on Derivative, Net $ 5.8 $ (1.4) $ 8.9 $ 0.5