Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 1,790.1
Other comprehensive income (loss) before reclassifications	(129.1)	$ (104.5)
Amounts reclassified from AOCI	6.4	10.7
Other comprehensive income (loss)	(122.7)	(93.8)
Ending balance	1,692.9	1,477.6
Pretax reclassification amounts from AOCI, tax	(1.7)	(2.9)
Net Unrealized Investment Gains (Losses) on Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	366.3	230.4
Other comprehensive income (loss) before reclassifications	(129.1)	(104.5)
Amounts reclassified from AOCI	6.4	10.7
Other comprehensive income (loss)	(122.7)	(93.8)
Ending balance	243.6	136.6
Pretax reclassification amounts from accumulated other comprehensive income (loss)	(8.1)	(13.6)
Net Funded Status of Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(11.2)	(10.7)
Other comprehensive income (loss) before reclassifications	0.0	0.0
Amounts reclassified from AOCI	0.0	0.0
Other comprehensive income (loss)	0.0	0.0
Ending balance	(11.2)	(10.7)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	355.1	219.7
Ending balance	$ 232.4	$ 125.9