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EMPLOYEE BENEFIT PLANS (Tables)
 3 Months Ended
Mar. 31, 2015
Compensation and Retirement Disclosure [Abstract]  
Schedule of the components of net periodic benefit cost
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 March 31
 
Three Months Ended March 31
(Millions)
 
2015
 
2014
 
2015
 
2014
Service cost
 
$
7.6

 
$
6.6

 
$
5.8

 
$
5.9

Interest cost
 
17.0

 
19.7

 
5.3

 
7.1

Expected return on plan assets
 
(26.9
)
 
(28.9
)
 
(7.9
)
 
(8.8
)
Loss on plan settlement
 
0.5

 
—

 
—

 
—

Amortization of prior service cost (credit)
 
0.1

 
0.2

 
(2.6
)
 
(1.3
)
Amortization of net actuarial loss
 
10.5

 
8.4

 
1.1

 
0.7

Net periodic benefit cost
 
$
8.8

 
$
6.0

 
$
1.7

 
$
3.6