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ARO Trust (Tables)
9 Months Ended
Sep. 30, 2017
Investments, Debt and Equity Securities [Abstract]  
ARO Trust
Investments in available-for-sale securities within the ARO Trust at fair value were as follows (in millions): 
 
September 30, 2017
 
December 31, 2016
 
Amortized
Cost Basis
 
Fair
Value
 
Amortized
Cost Basis
 
Fair
Value
Cash and Money Market Funds
$
8.5

 
$
8.5

 
$
5.0

 
$
5.0

U.S. Equity Funds
35.9

 
47.7

 
29.4

 
36.5

International Equity Funds
20.7

 
23.7

 
19.2

 
18.6

Municipal Bond Funds
46.8

 
47.1

 
36.7

 
36.3

Total
$
111.9

 
$
127.0

 
$
90.3

 
$
96.4