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ARO Trust (Tables)
9 Months Ended
Sep. 30, 2016
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, 2016
 
December 31, 2015
 
Amortized
Cost Basis
 
Fair
Value
 
Amortized
Cost Basis
 
Fair
Value
Cash and Money Market Funds
$
3.0

 
$
3.0

 
$
3.2

 
$
3.2

U.S. Equity Funds
29.4

 
35.3

 
19.2

 
22.9

International Equity Funds
19.2

 
19.1

 
16.1

 
15.0

Municipal Bond Funds
34.7

 
35.7

 
25.1

 
25.6

Total
$
86.3

 
$
93.1

 
$
63.6

 
$
66.7