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ARO Trust (Tables)
 12 Months Ended
Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]  
ARO Trust
Investments within the ARO Trust at fair value were as follows (in millions):
 
 
 
December 31, 2019
 
December 31, 2018
 
 
Amortized
Cost Basis
 
Fair
Value
 
Amortized
Cost Basis
 
Fair
Value
Money Market Funds
 
$
15.8

 
$
15.8

 
$
21.7

 
$
21.7

U.S. Equity Funds
 
55.3

 
83.3

 
46.4

 
56.8

International Equity Funds
 
31.8

 
35.4

 
21.9

 
21.4

Municipal Bond Funds
 
64.7

 
66.1

 
50.1

 
49.6

Total
 
$
167.6

 
$
200.6

 
$
140.1

 
$
149.5