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ARO Trust (Tables)
12 Months Ended
Dec. 31, 2015
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):
 
 
 
December 31, 2015
 
December 31, 2014
 
 
Amortized
Cost Basis
 
Fair
Value
 
Amortized
Cost Basis
 
Fair
Value
Cash and Money Market Funds
 
$
3.2

 
$
3.2

 
$
2.1

 
$
2.1

U.S. Equity Funds
 
19.2

 
22.9

 
15.0

 
19.0

International Equity Funds
 
16.1

 
15.0

 
8.0

 
8.2

Municipal Bond Funds
 
25.1

 
25.6

 
17.7

 
18.2

Total
 
$
63.6

 
$
66.7

 
$
42.8

 
$
47.5