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ARO Trust (Tables)
3 Months Ended
Mar. 31, 2014
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, 2014
 
December 31, 2013
 
Amortized
Cost Basis
 
Fair
Value
 
Amortized
Cost Basis
 
Fair
Value
Cash and Money Market Funds
$
13.9

 
$
13.9

 
$
6.5

 
$
6.5

U.S. Equity Funds
6.7

 
9.3

 
8.0

 
11.1

International Equity Funds
5.7

 
6.4

 
4.2

 
4.9

Municipal Bond Funds
15.2

 
15.4

 
10.2

 
10.2

Total
$
41.5

 
$
45.0

 
$
28.9

 
$
32.7