Schedule II - Parent Only Condensed Financial Statements - Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Condensed Financial Statements, Captions [Line Items]
Net income (loss)	$ 386.8	$ 252.6	$ 519.6
Other Comprehensive Income (Loss), Tax [Abstract]
Change in unrealized gains on securities, net of taxes of $(4.3), $3.0 and $(4.3) for the years ended December 31, 2019, 2018 and 2017, respectively	555.5	(342.3)	121.9
Change in unrealized gains on derivative transactions, net of taxes of $0.4 and $(4.9) for the years ended December 31, 2019 and 2018, respectively	(1.3)	18.4	0.0
Change in foreign currency translation, net of taxes of $0.0, $0.0 and $0.1 for the years ended December 31, 2019, 2018 and 2017, respectively	16.7	(94.2)	40.6
Amortization of pension and postretirement unrecognized net periodic benefit cost and change in funded status, net of taxes of $1.1, $3.4 and $11.0 for the years ended December 31, 2019, 2018 and 2017, respectively	(4.4)	(12.7)	(20.4)
Total other comprehensive (loss) income	566.9	(437.5)	139.4
Total comprehensive (loss) income	953.7	(184.9)	659.0
Less: Comprehensive income attributable to non-controlling interest	(4.2)	(1.6)	0.0
Total comprehensive (loss) income attributable to common stockholders	949.5	(186.5)	659.0
Parent Company
Condensed Financial Statements, Captions [Line Items]
Net income (loss)	386.8	252.6	519.6
Other Comprehensive Income (Loss), Tax [Abstract]
Change in unrealized gains on securities, net of taxes of $(4.3), $3.0 and $(4.3) for the years ended December 31, 2019, 2018 and 2017, respectively	16.3	(11.3)	2.5
Change in unrealized gains on derivative transactions, net of taxes of $0.4 and $(4.9) for the years ended December 31, 2019 and 2018, respectively	(1.3)	18.4	0.0
Change in foreign currency translation, net of taxes of $0.0, $0.0 and $0.1 for the years ended December 31, 2019, 2018 and 2017, respectively	0.0	0.0	(0.1)
Amortization of pension and postretirement unrecognized net periodic benefit cost and change in funded status, net of taxes of $1.1, $3.4 and $11.0 for the years ended December 31, 2019, 2018 and 2017, respectively	(4.2)	(12.7)	(20.4)
Change in subsidiary other comprehensive income	556.1	(431.9)	157.4
Total other comprehensive (loss) income	566.9	(437.5)	139.4
Total comprehensive (loss) income	953.7	(184.9)	659.0
Less: Comprehensive income attributable to non-controlling interest	(4.2)	(1.6)	0.0
Total comprehensive (loss) income attributable to common stockholders	$ 949.5	$ (186.5)	$ 659.0