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Benefit Plans (Tables)
 3 Months Ended
Mar. 31, 2017
Compensation and Retirement Disclosure [Abstract]  
Company's pension and postretirement benefit plans
The components of net periodic cost for the Company’s pension and postretirement benefit plans are as follows:
($ in millions)
Three months ended March 31,
 
2017
 
2016
Pension benefits
 
 
 
Service cost
$
29

 
$
28

Interest cost
66

 
71

Expected return on plan assets
(102
)
 
(99
)
Amortization of:
 

 
 

Prior service credit
(14
)
 
(14
)
Net actuarial loss
47

 
43

Settlement loss
8

 
8

Net periodic pension cost
$
34

 
$
37

 
 
 
 
Postretirement benefits
 
 
 
Service cost
$
2

 
$
2

Interest cost
4

 
4

Amortization of:
 
 
 
Prior service credit
(6
)
 
(5
)
Net actuarial gain
(6
)
 
(8
)
Net periodic postretirement credit
$
(6
)
 
$
(7
)