Retirement Benefits (Details 7) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss), before Tax [Roll Forward]
Amortization of net actuarial loss	$ 71.6	$ 75.0	$ 75.0
Amortization to net income (loss) of net prior service cost (credits)	(1.6)	(0.4)	6.2
Net loss arising during the period	(42.7)	(92.1)	(95.8)
Pension Benefits [Member]
Accumulated Other Comprehensive Income (Loss), before Tax [Roll Forward]
Beginning of year accumulated other comprehensive loss	(1,462.7)	(1,418.2)
Amortization of net actuarial loss	62.6	65.4
Amortization to net income (loss) of net prior service cost (credits)	1.3	1.3
Net loss arising during the period	(27.3)	(111.2)
End of year accumulated other comprehensive loss	(1,426.1)	(1,462.7)	(1,418.2)
Net change in accumulated other comprehensive loss	36.6	(44.5)
Other Postretirement Benefits [Member]
Accumulated Other Comprehensive Income (Loss), before Tax [Roll Forward]
Beginning of year accumulated other comprehensive loss	(74.4)	(101.4)
Amortization of net actuarial loss	9.0	9.6
Amortization to net income (loss) of net prior service cost (credits)	(2.9)	(1.7)
Net loss arising during the period	(15.4)	19.1
End of year accumulated other comprehensive loss	(83.7)	(74.4)	$ (101.4)
Net change in accumulated other comprehensive loss	$ (9.3)	$ 27.0