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EMPLOYEE BENEFIT PLANS (Tables)
 3 Months Ended
Jun. 30, 2013
Compensation and Retirement Disclosure [Abstract]  
Schedule of components of net periodic benefit cost for benefit plans
The following table shows the components of net periodic benefit cost (including amounts capitalized to our balance sheets) for our benefit plans:
 
 
Pension Benefits
 
Other Postretirement Benefits
 
 
Three Months Ended
June 30
 
Six Months Ended
June 30
 
Three Months Ended
June 30
 
Six Months Ended
June 30
(Millions)
 
2013
 
2012
 
2013
 
2012
 
2013
 
2012
 
2013
 
2012
Service cost
 
$
2.7

 
$
3.0

 
$
5.4

 
$
6.4

 
$
2.6

 
$
2.1

 
$
5.3

 
$
4.3

Interest cost
 
7.5

 
8.3

 
15.3

 
17.0

 
3.4

 
3.8

 
6.7

 
7.5

Expected return on plan assets
 
(14.2
)
 
(13.8
)
 
(28.6
)
 
(27.7
)
 
(3.7
)
 
(3.7
)
 
(7.4
)
 
(7.3
)
Amortization of transition obligation
 
—

 
—

 
—

 
—

 
—

 
—

 
—

 
0.1

Amortization of prior service cost (credit)
 
0.9

 
1.2

 
1.8

 
2.3

 
(0.5
)
 
(0.7
)
 
(1.0
)
 
(1.5
)
Amortization of net actuarial loss
 
6.2

 
3.8

 
12.0

 
7.4

 
1.9

 
1.5

 
3.7

 
2.8

Net periodic benefit cost
 
$
3.1

 
$
2.5

 
$
5.9

 
$
5.4

 
$
3.7

 
$
3.0

 
$
7.3

 
$
5.9