Consolidated Statements of Comprehensive Income (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Unrealized gains net of tax provision of $4.0, tax benefit of $0.7, and tax provision of $6.5 for 2017, 2016, and 2015, respectively	$ (4.0)	$ 0.7	$ (6.5)
Reclassification adjustments for realized net gains included in net income, net of tax provisions of $0.9 and $0.5, and zero for 2017, 2016, and 2015, respectively	0.9	0.5	0.0
Unrealized gain (losses) net of tax provisions of $4.4, $0.8, and $0.4 for 2017, 2016, and 2015, respectively	(4.4)	(0.8)	(0.4)
Reclassification adjustments for realized (gains) loss included in net income, net of tax provisions of $2.4 and $0.7, and zero for 2017, 2016, and 2015, respectively	$ 2.4	$ 0.7	$ 0.0