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Retirement Benefits (Unaudited) (Tables)
 3 Months Ended
Mar. 31, 2018
Retirement Benefits [Abstract]  
Components of net periodic benefit cost
The cost to the company of its retirement plans is shown in the following table:
 
Three Months Ended March 31
 
Pension
Benefits
 
Medical and
Life Benefits
$ in millions
2018
 
2017
 
2018
 
2017
Components of net periodic benefit cost
 
 
 
 
 
 
 
Service cost
$
99

 
$
97

 
$
5

 
$
5

Interest cost
290

 
313

 
19

 
21

Expected return on plan assets
(529
)
 
(471
)
 
(25
)
 
(22
)
Amortization of:
 
 
 
 
 
 
 
Prior service credit
(15
)
 
(15
)
 
(5
)
 
(5
)
Net loss from previous years
134

 
191

 
—

 
3

Net periodic benefit cost
$
(21
)
 
$
115

 
$
(6
)
 
$
2
Employer contributions to retirement plans
Contributions made by the company to its retirement plans are as follows:
 
Three Months Ended March 31
$ in millions
2018
 
2017
Defined benefit pension plans
$
22

 
$
23

Medical and life benefit plans
11

 
11

Defined contribution plans
104

 
99