Income Taxes - Reconciliation of unrecognized tax benefit (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefit as of January 1	$ 5.9	$ 3.9	$ 3.9
Changes for tax positions of prior years	3.8	2.8	0.6
Increases for tax positions related to the current year	0.4	0.6	0.1
Decreases relating to settlements and lapsing of statutes of limitations	(4.9)	(1.4)	(0.7)
Unrecognized tax benefit as of December 31	$ 5.2	$ 5.9	$ 3.9