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Employee Retirement Plans (Tables)
 6 Months Ended
Jun. 30, 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 June 30,
 
Six months ended June 30,
(millions)
2015
 
2014
 
2015
 
2014
Pension:
 
 
 
 
 
 
 
Service cost of benefits earned
$
12

 
$
9

 
$
25

 
$
18

Interest cost on projected benefit obligation
17

 
17

 
34

 
33

Expected return on plan assets
(21
)
 
(20
)
 
(42
)
 
(40
)
Net amortization
10

 
6

 
19

 
12

     Net pension cost
$
18

 
$
12

 
$
36

 
$
23

Postretirement:
 
 
 
 
 
 
 
Service cost of benefits earned
$
—

 
$
—

 
$
1

 
$
1

Interest cost on projected benefit obligation
2

 
2

 
3

 
4

Net amortization
(8
)
 
(9
)
 
(16
)
 
(18
)
     Net postretirement benefit
$
(6
)
 
$
(7
)
 
$
(12
)
 
$
(13
)