Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Cash Flows From Operating Activities
Net income	$ 35.3	$ 116.0	$ 422.8
Adjustments to reconcile net income to net cash provided by operating activities:
Net realized investment (gains) losses	32.5	106.5	(123.0)
Net amortization and depreciation	6.3	12.9	16.9
Stock-based compensation expense	31.1	29.0	22.9
Amortization of defined benefit plan costs	7.9	5.5	3.4
Deferred income tax expense (benefit)	(24.6)	(46.5)	25.0
Change in deferred acquisition costs	(16.0)	(52.8)	(74.5)
Change in premiums receivable, net of reinsurance premiums payable	(110.3)	(116.7)	(123.4)
Change in loss, loss adjustment expense and unearned premium reserves	442.0	784.7	671.8
Change in reinsurance recoverable	(91.6)	(57.2)	(32.9)
Change in expenses and taxes payable	48.0	3.3	179.0
Other, net	1.3	(62.3)	(163.8)
Net cash provided by operating activities	361.7	722.3	823.7
Cash Flows From Investing Activities
Proceeds from disposals and maturities of fixed maturities	772.9	1,106.3	1,564.4
Proceeds from disposals of equity securities and other investments	141.1	488.4	286.3
Purchase of fixed maturities	(1,059.8)	(1,960.2)	(2,112.7)
Purchase of equity securities and other investments	(70.8)	(124.3)	(190.2)
Capital expenditures	(11.9)	(17.8)	(8.0)
Net cash used in investing activities	(228.5)	(507.6)	(460.2)
Cash Flows From Financing Activities
Proceeds from exercise of employee stock options	6.5	13.3	20.5
Dividends paid to shareholders	(117.2)	(108.9)	(102.2)
Repurchases of common stock		(30.8)	(162.6)
Other financing activities	(11.4)	(14.2)	(8.9)
Net cash used in financing activities	(122.1)	(140.6)	(253.2)
Net change in cash and cash equivalents	11.1	74.1	110.3
Cash and cash equivalents, beginning of year	305.0	230.9	120.6
Cash and cash equivalents, end of year	316.1	305.0	230.9
Supplemental Cash Flow Information
Interest payments	33.2	33.3	33.4
Income tax payments (refunds), net	(22.1)	107.8	74.0
Continuing Operations [Member]
Adjustments to reconcile net income to net cash provided by operating activities:
Net realized investment (gains) losses	$ 32.3	$ 106.4	$ (123.5)