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Derivative Financial Instruments - Information about Fair Values of Derivative Instruments in Consolidated Balance Sheet (Parenthetical) (Detail) - USD ($)
$ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Offsetting [Abstract]    
Reduction in estimated fair value of interest rate contracts in asset position $ 302.4 $ 136.2
Reduction in estimated fair value of interest rate contracts in liability position $ 2.7 $ 12.2