Business Segment Information (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) segment	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Business segment information
Number of reportable segments | segment			2
Revenue	$ 521.1	$ 466.6	$ 1,472.7	$ 1,342.0
Operating Income (loss)	87.8	69.1	183.2	156.0
Assets	2,352.1		2,352.1		$ 2,128.6
Eliminations
Business segment information
Revenue	(3.2)	(1.4)	(10.5)	(3.7)
Corporate, eliminations and other
Business segment information
Operating Income (loss)	0.1	0.1		0.2
Assets	(3.6)		(3.6)		(5.2)
BSI | Operating segments
Business segment information
Revenue	471.8	417.1	1,331.0	1,206.5
Operating Income (loss)	84.0	63.6	173.3	146.5
Assets	2,296.5		2,296.5		2,100.6
BEST | Operating segments
Business segment information
Revenue	52.5	50.9	152.2	139.2
Operating Income (loss)	3.7	$ 5.4	9.9	$ 9.3
Assets	$ 59.2		$ 59.2		$ 33.2