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EMPLOYEE BENEFIT PLANS
 9 Months Ended
Sep. 30, 2013
Compensation and Retirement Disclosure [Abstract]  
EMPLOYEE BENEFIT PLANS
EMPLOYEE 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
September 30
 
Nine Months Ended
September 30
 
Three Months Ended
September 30
 
Nine Months Ended
September 30
(Millions)
 
2013
 
2012
 
2013
 
2012
 
2013
 
2012
 
2013
 
2012
Service cost
 
$
2.7

 
$
3.2

 
$
8.1

 
$
9.6

 
$
2.6

 
$
2.1

 
$
7.9

 
$
6.4

Interest cost
 
7.6

 
8.5

 
22.9

 
25.5

 
3.4

 
3.8

 
10.1

 
11.3

Expected return on plan assets
 
(14.3
)
 
(13.8
)
 
(42.9
)
 
(41.5
)
 
(3.7
)
 
(3.6
)
 
(11.1
)
 
(10.9
)
Amortization of transition obligation
 
—

 
—

 
—

 
—

 
—

 
0.1

 
—

 
0.2

Amortization of prior service cost (credit)
 
0.9

 
1.1

 
2.7

 
3.4

 
(0.6
)
 
(0.8
)
 
(1.6
)
 
(2.3
)
Amortization of net actuarial loss
 
6.0

 
3.7

 
18.0

 
11.1

 
1.9

 
1.4

 
5.6

 
4.2

Net periodic benefit cost
 
$
2.9

 
$
2.7

 
$
8.8

 
$
8.1

 
$
3.6

 
$
3.0

 
$
10.9

 
$
8.9



Transition obligations, prior service costs (credits), and net actuarial losses that have not yet been recognized as a component of net periodic benefit cost are recorded as net regulatory assets.

We make contributions to our plans in accordance with legal and tax requirements. These contributions do not necessarily occur evenly throughout the year. During the nine months ended September 30, 2013, we contributed $38.0 million to our pension plans and $0.1 million to our other postretirement benefit plans. We expect to contribute an additional $0.8 million to our pension plans and $15.6 million to our other postretirement benefit plans during the remainder of 2013, dependent upon various factors affecting us, including our liquidity position and tax law changes.