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Other Postretirement Employee Benefit Plans (Tables) - Other postretirement benefits
 12 Months Ended
Dec. 31, 2017
Defined Benefit Plans and Other Post-Retirement Benefit Plans [Line Items]  
Funded Status Of Pension Plans
The following sets forth the funded status of the domestic plans as of the most recent actuarial valuations using measurement dates of December 31 ($ in millions):
 
2017
 
2016
Change in benefit obligation:
 
 
 
Benefit obligation at beginning of year
$
174.6

 
$
193.4

Service cost
0.7

 
0.7

Interest cost
5.6

 
6.6

Amendments, curtailments and other
0.4

 
(6.5
)
Actuarial loss (gain)
1.5

 
(5.0
)
Retiree contributions
2.9

 
3.2

Benefits paid
(18.4
)
 
(17.8
)
Benefit obligation at end of year
167.3

 
174.6

Change in plan assets:
 
 
 
Fair value of plan assets
—

 
—

Funded status
$
(167.3
)
 
$
(174.6
)
Weighted Average Assumptions Used To Determine Benefit Obligations And Cost
Weighted average assumptions used to determine benefit obligations at date of measurement:
 
2017
 
2016
Discount rate
3.5
%
 
3.9
%
Medical trend rate – initial
6.3
%
 
6.5
%
Medical trend rate – grading period
20 years

 
21 years

Medical trend rate – ultimate
4.5
%
 
4.5
%
Effect Of One-Percentage-Point Change In Assumed Health Care Cost Trend Rates
Effect of a one-percentage-point change in assumed health care cost trend rates:
($ in millions)
1% Increase
 
1% Decrease
Effect on the total of service and interest cost components
$
0.3

 
$
(0.3
)
Effect on postretirement medical benefit obligation
4.9

 
(4.3
)
Components of Net Periodic Pension Cost
Components of net periodic benefit cost:
($ in millions)
2017
 
2016
Service cost
$
0.7

 
$
0.7

Interest cost
5.6

 
6.6

Amortization of net gain
(0.1
)
 
—

Amortization of prior service credit
(3.1
)
 
(3.1
)
Net periodic benefit cost
$
3.1

 
$
4.2
Benefit Payments That Reflect Expected Future Service
The following sets forth benefit payments, which reflect expected future service, as appropriate, expected to be paid in the periods indicated ($ in millions):
2018
$
15.8

2019
15.2

2020
14.6

2021
14.0

2022
13.3

2023 – 2027
57.8