Hedging Transactions and Derivative Financial Instruments - Summary of Reconciliation of Company's Derivatives by Contract Type (Parenthetical) (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Aggregate margin account balances included in cash and cash equivalents	$ 3.1	$ 5.3