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Retirement Benefits (Tables)
 3 Months Ended
Mar. 31, 2017
Compensation and Retirement Disclosure [Abstract]  
Schedule of Components of Retirement Benefit Expense (Benefit)
Components of retirement benefit expense (benefit) are: 
 
Pension Benefits
 
Postretirement Medical and Life
Insurance Benefits
 
Three months ended March 31,
 
2017
 
2016
 
2017
 
2016
 
(In millions)
Service cost
$
3.7

 
$
3.5

 
$
—

 
$
—

Interest cost on benefit obligation
14.4

 
16.0

 
0.4

 
0.5

Assumed return on plan assets
(16.1
)
 
(17.5
)
 
—

 
—

Amortization of prior service credits
—

 
—

 
(0.1
)
 
(0.3
)
Recognized net actuarial losses (gains)
17.0

 
15.9

 
(1.0
)
 
(0.9
)
Retirement benefit expense (benefit)
$
19.0

 
$
17.9

 
$
(0.7
)
 
$
(0.7
)