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ARO Trust (Tables)
3 Months Ended
Mar. 31, 2018
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): 
 
March 31, 2018
 
December 31, 2017
 
Amortized
Cost Basis
 
Fair
Value
 
Amortized
Cost Basis
 
Fair
Value
Cash and Money Market Funds
$
24.3

 
$
24.3

 
$
12.6

 
$
12.6

U.S. Equity Funds
35.9

 
50.0

 
35.9

 
50.5

International Equity Funds
20.7

 
24.4

 
20.7

 
24.6

Municipal Bond Funds
46.8

 
46.3

 
46.8

 
46.9

Total
$
127.7

 
$
145.0

 
$
116.0

 
$
134.6