Central Merger and Restructuring, Merger and Integration Costs (Tables)

12 Months Ended

Dec. 31, 2017

Business Combinations [Abstract]

Schedule of acquisition related costs

	Year Ended December 31,
(millions)	2017		2016		2015
General and administrative expenses	$	1.2	$	4.5	$	6.2
Depreciation and amortization	—		2.4		1.0
Total	$	1.2	$	6.9	$	7.2