Postretirement Benefit Plans (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Components of net periodic benefit cost:
Service cost	$ 3.5	$ 3.7
Interest cost	8.0	9.5
Expected return on plan assets	(9.7)	(10.1)
Amortization of prior service credit	0.3	0.4
Net periodic benefit cost	6.5	3.5
Postretirement Benefit Plans [Member]
Components of net periodic benefit cost:
Service cost	0.0	0.1
Interest cost	2.3	2.7
Expected return on plan assets	(1.4)	(1.6)
Amortization of prior service credit	(0.2)	0.2
Net periodic benefit cost	$ 0.7	$ 1.4