Condensed Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 46.0	$ 30.9	$ 82.7	$ 58.6
Other comprehensive income:
Foreign currency translation adjustments	(1.2)	2.3	(0.6)	3.8
Unrealized gains on securities:
Gains arising during the year	27.5	39.1	61.6	77.9
Reclassification adjustment for gains included in net income	(2.7)	(0.3)	(18.9)	(8.1)
Other comprehensive income before tax	23.6	41.1	42.1	73.6
Unrealized gains on securities:
Gains arising during the year	6.1	12.1	15.9	19.9
Reclassification adjustment for gains included in net income	(1.0)	0.2	(6.2)	(3.9)
Income tax provision related to other comprehensive income	5.1	12.3	9.7	16.0
Other comprehensive income, net of tax	18.5	28.8	32.4	57.6
Comprehensive income	$ 64.5	$ 59.7	$ 115.1	$ 116.2