Business Segment Information (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) segment	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Business segment information
Number of reportable segments | segment			2
Revenue	$ 490.2	$ 443.7	$ 951.6	$ 875.4
Operating Income (loss)	53.5	48.8	95.4	86.9
Assets	2,349.7		2,349.7		$ 2,128.6
Eliminations
Business segment information
Revenue	(4.1)	(1.4)	(7.3)	(2.3)
Corporate, eliminations and other
Business segment information
Operating Income (loss)	(0.3)	(0.1)	(0.1)	0.1
Assets	(3.6)		(3.6)		(5.2)
BSI | Operating segments
Business segment information
Revenue	442.4	402.4	859.2	789.4
Operating Income (loss)	50.7	47.2	89.3	82.9
Assets	2,296.6		2,296.6		2,100.6
BEST | Operating segments
Business segment information
Revenue	51.9	42.7	99.7	88.3
Operating Income (loss)	3.1	$ 1.7	6.2	$ 3.9
Assets	$ 56.7		$ 56.7		$ 33.2