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Employee Retirement Plans (Tables)
 3 Months Ended
Mar. 31, 2015
Compensation and Retirement Disclosure [Abstract]  
Components of net pension and postretirement benefits costs
The components of net pension and postretirement benefits costs are summarized in the following table:
 
Three months ended March 31,
(millions)
2015
 
2014
Pension:
 
 
 
Service cost of benefits earned
$
13

 
$
9

Interest cost on projected benefit obligation
17

 
16

Expected return on plan assets
(21
)
 
(20
)
Net amortization
9

 
6

     Net pension cost
$
18

 
$
11

Postretirement:
 
 
 
Service cost of benefits earned
$
1

 
$
1

Interest cost on projected benefit obligation
1

 
2

Net amortization
(8
)
 
(9
)
     Net postretirement benefit
$
(6
)
 
$
(6
)