Postretirement Benefit Plans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Components of net periodic benefit cost:
Service cost	$ 3.4	$ 3.6	$ 6.9	$ 7.3
Interest cost	8.0	9.5	16.0	19.0
Expected return on plan assets	(9.7)	(10.2)	(19.4)	(20.3)
Amortization of prior service credit	0.4	0.4	0.7	0.8
Net periodic benefit cost	2.1	3.3	8.6	6.8
Postretirement Benefit Plans [Member]
Components of net periodic benefit cost:
Service cost	0.1	0.1	0.1	0.2
Interest cost	2.2	2.8	4.5	5.5
Expected return on plan assets	(1.4)	(1.7)	(2.8)	(3.3)
Amortization of prior service credit	(0.3)	0.3	(0.5)	0.5
Net periodic benefit cost	$ 0.6	$ 1.5	$ 1.3	$ 2.9