Employee Benefit Plans (Fair Value Measurements Classified as Level 3) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Changes in Plan Assets:
Beginning fair value of plan assets	$ 164.9
Ending fair value of plan assets	178.8	$ 164.9
Significant Unobservable Inputs (Level 3)
Changes in Plan Assets:
Beginning fair value of plan assets	3.2
Ending fair value of plan assets	2.8	3.2
Coal Lease
Changes in Plan Assets:
Beginning fair value of plan assets	3.2
Ending fair value of plan assets	2.8	3.2
Coal Lease | Significant Unobservable Inputs (Level 3)
Changes in Plan Assets:
Beginning fair value of plan assets	3.2	3.5
Change in fair value of plan assets	(0.4)	(0.3)
Ending fair value of plan assets	$ 2.8	$ 3.2