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Debt Unrealized Gains (Losses) From Debt Hedges Included In Accumulated Other (Detail) - USD ($)
$ in Millions
Dec. 31, 2017
Apr. 06, 2017
Dec. 31, 2016
Jan. 26, 2015
Apr. 22, 2014
Aug. 17, 2011
Nov. 21, 2002
Mar. 01, 1999
Debt Instrument [Line Items]                
Unrealized Gain (Loss) $ (14.8)   $ (9.4)          
3.75% Senior Notes due 2021                
Debt Instrument [Line Items]                
Unrealized Gain (Loss) (2.1)         $ (5.1)    
6 5/8% Senior Notes due 2029                
Debt Instrument [Line Items]                
Unrealized Gain (Loss) (2.7)             $ (4.2)
6.25% Senior Notes due 2032                
Debt Instrument [Line Items]                
Unrealized Gain (Loss) 3.6           $ 5.1  
4.35% Senior Notes due 2044                
Debt Instrument [Line Items]                
Unrealized Gain (Loss) (1.5)       $ (1.6)      
3.70% Senior Notes due 2045                
Debt Instrument [Line Items]                
Unrealized Gain (Loss) (12.2)     $ (12.9)        
4.125% Senior Notes due 2047                
Debt Instrument [Line Items]                
Unrealized Gain (Loss) $ (7.9) $ (8.0)