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Postretirement Benefit Plans (Tables)
 6 Months Ended
Jun. 30, 2018
Postretirement Benefit Plans [Member]  
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]  
Schedule of Defined Benefit Plans Disclosures [Table Text Block] The following table sets forth the net periodic benefit cost for the Company’s other postretirement benefit plans. The amounts for the three and six months ended June 30, 2018 are based on calculations prepared by the Company's actuaries and represent the Company’s best estimate of the respective period’s proportionate share of the amounts to be recorded for the year ending December 31, 2018.
 
Three Months Ended
June 30,
Six Months Ended
June 30,
 
2018
2017
2018
2017
Components of net periodic benefit cost:
 
 
 
 
Service cost
$
0.1

$
0.1

$
0.1

$
0.1

Interest cost
1.9

2.2

3.7

4.5

Expected return on plan assets
(1.0
)
(1.4
)
(1.9
)
(2.8
)
Amortization of prior service credit
(0.4
)
(0.3
)
(0.8
)
(0.5
)
   Net periodic benefit cost
$
0.6

$
0.6

$
1.1

$
1.3