Consolidated Statements of Comprehensive Income (unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 142.6	$ 67.0	$ 311.2	$ 173.0
Other comprehensive income (loss):
Change in unrealized gains on securities, net of taxes of $(60.0), $28.2, $(129.0) and $76.0 for the three and six months ended June 30, 2019 and 2018, respectively	228.5	(105.9)	477.4	(281.1)
Change in unrealized gains on derivative transactions, net of taxes of $0.1, $0.2, $0.3 and $(5.4) for the three and six months ended June 30, 2019 and 2018, respectively	(2.9)	(0.2)	(3.2)	20.9
Change in other-than-temporary impairment losses, net of taxes of $(0.2), $0.4, $(0.1) and $1.3 for the three and six months ended June 30, 2019 and 2018, respectively	0.7	(1.4)	0.5	(4.9)
Change in foreign currency translation, net of taxes of $(0.8,) $1.0, $(1.4) and $1.5 for the three and six months ended June 30, 2019 and 2018, respectively	(1.2)	(83.1)	9.0	(73.9)
Amortization of pension and postretirement unrecognized net periodic benefit cost, net of taxes of $(0.1), $(0.3), $(0.1) and $(0.3) for the three and six months ended June 30, 2019 and 2018, respectively	0.2	1.0	0.4	1.0
Total other comprehensive income (loss)	225.3	(189.6)	484.1	(338.0)
Total comprehensive income (loss)	367.9	(122.6)	795.3	(165.0)
Less: Comprehensive loss (income) attributable to non-controlling interests	1.5	0.0	(1.4)	0.0
Total comprehensive income (loss) attributable to stockholders	$ 369.4	$ (122.6)	$ 793.9	$ (165.0)