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ARO Trust (Tables)
6 Months Ended
Jun. 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): 
 
June 30, 2017
 
December 31, 2016
 
Amortized
Cost Basis
 
Fair
Value
 
Amortized
Cost Basis
 
Fair
Value
Cash and Money Market Funds
$
13.0

 
$
13.0

 
$
5.0

 
$
5.0

U.S. Equity Funds
32.3

 
42.5

 
29.4

 
36.5

International Equity Funds
19.2

 
21.0

 
19.2

 
18.6

Municipal Bond Funds
42.8

 
43.0

 
36.7

 
36.3

Total
$
107.3

 
$
119.5

 
$
90.3

 
$
96.4