Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			$ 1,790.1
Other comprehensive income (loss) before reclassifications	$ 93.3	$ 147.5	(35.8)	$ 43.0
Amounts reclassified from AOCI	(4.7)	(5.0)	1.7	5.7
Other comprehensive income (loss)	88.6	142.5	(34.1)	48.7
Ending balance	1,816.6	1,639.8	1,816.6	1,639.8
Pretax reclassification amounts from AOCI, tax	1.2	1.4	(0.5)	(1.5)
Net Unrealized Investment Gains (Losses) on Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	243.6	136.6	366.3	230.4
Other comprehensive income (loss) before reclassifications	93.3	147.5	(35.8)	43.0
Amounts reclassified from AOCI	(4.7)	(5.0)	1.7	5.7
Other comprehensive income (loss)	88.6	142.5	(34.1)	48.7
Ending balance	332.2	279.1	332.2	279.1
Pretax reclassification amounts from accumulated other comprehensive income (loss)	5.9	6.4	(2.2)	(7.2)
Net Funded Status of Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(11.2)	(10.7)	(11.2)	(10.7)
Other comprehensive income (loss) before reclassifications	0.0	0.0	0.0	0.0
Amounts reclassified from AOCI	0.0	0.0	0.0	0.0
Other comprehensive income (loss)	0.0	0.0	0.0	0.0
Ending balance	(11.2)	(10.7)	(11.2)	(10.7)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	232.4	125.9	355.1	219.7
Ending balance	$ 321.0	$ 268.4	$ 321.0	$ 268.4