Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 63.6	$ 50.3	$ 146.7
Other comprehensive income:
Foreign currency translation adjustments	(3.4)	(1.4)	4.0
Defined benefit pension plans:
Net gain (loss) arising during the year	1.2	1.2	(0.3)
Unrealized (losses) gains on securities:
(Losses) gains arising during the year	(93.9)	105.7	58.1
Reclassification adjustment for loss (gains) included in net income	5.4	(41.6)	(23.2)
Other comprehensive (loss) income before tax	(90.7)	63.9	38.6
Defined benefit pension plans:
Net gain (loss) arising during the year	0.2	0.4	(0.1)
Unrealized (losses) gains on securities:
(Losses) gains arising during the year	(13.5)	28.0	15.7
Reclassification adjustment for losses (gains) included in net income	0.5	(13.4)	(13.2)
Income tax (benefit) provision related to other comprehensive income	(12.8)	15.0	2.4
Other comprehensive (loss) income, net of tax	(77.9)	48.9	36.2
Comprehensive (loss) income	$ (14.3)	$ 99.2	$ 182.9