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Stock-Based Compensation Phantom Stock Units Summary (Details) - Phantom Share Units (PSUs) [Member] - USD ($)
$ / shares in Units, shares in Thousands, $ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized $ 5.7    
Outstanding, beginning balance, Number 63 62  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value $ 62.61   $ 51.88
Shares converted upon acquisition, number 21    
Shares converted upon acquisition, Weighted Average Grant Date Fair Value $ 69.40    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period 31 32  
Weighted average fair value $ 69.30 $ 65.55  
Stock Issued During Period, Shares, Share-based Compensation, Net of Forfeitures (27) (19)  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value $ 50.19    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period (1) (2)  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value $ 69.36    
Outstanding, ending balance, Number 87 73  
Expected to Vest [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Outstanding, ending balance, Number 85 73