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Supplemental Financial Information - Noncash Investing and Financing Activities (Parenthetical) (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]      
Interest payments, net of amounts capitalized $ 97.0 $ 86.1 $ 94.7
Restricted Stock [Member]      
Condensed Financial Statements, Captions [Line Items]      
Number of shares of common stock withheld 29,416 7,923