Condensed Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 41.8	$ 46.0	$ 66.6	$ 82.7
Other comprehensive (loss) income:
Foreign currency translation adjustments	(2.9)	(1.2)	(4.0)	(0.6)
Unrealized (losses) gains on securities:
(Losses) gains arising during the year	(36.7)	27.5	(61.4)	61.6
Reclassification adjustment for losses (gains) included in net income	15.1	(2.7)	2.6	(18.9)
Other comprehensive (loss) income before tax	(24.5)	23.6	(62.8)	42.1
Unrealized (losses) gains on securities:
(Losses) gains arising during the year	(5.3)	6.1	(9.3)	15.9
Reclassification adjustment for losses (gains) included in net income	2.5	(1.0)	0.2	(6.2)
Income tax (benefit) provision related to other comprehensive income	(2.8)	5.1	(9.1)	9.7
Other comprehensive (loss) income, net of tax	(21.7)	18.5	(53.7)	32.4
Comprehensive income	$ 20.1	$ 64.5	$ 12.9	$ 115.1