Hedging Transactions and Derivative Financial Instruments - Summary of Reconciliation of Company's Derivatives by Contract Type (Parenthetical) (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Aggregate margin account balances included in cash and cash equivalents	$ 4.1	$ 9.2