Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash Flows From Operating Activities
Net income	$ 358.7	$ 425.1	$ 391.0
Adjustments to reconcile net income to net cash provided by operating activities:
Loss on repayment of debt	6.2		28.2
(Gain) loss from sale of Chaucer business		1.2	(131.9)
Net realized investment (gains) losses	(5.0)	(109.4)	50.7
Net amortization and depreciation	18.2	21.6	24.6
Stock-based compensation expense	20.1	17.4	13.4
Amortization of defined benefit plan costs	6.0	11.4	9.7
Deferred income tax expense (benefit)	(23.0)	12.5	(23.4)
Change in deferred acquisition costs	(10.2)	(14.4)	(19.2)
Change in premiums receivable, net of reinsurance premiums payable	(84.0)	(78.7)	(73.9)
Change in loss, loss adjustment expense and unearned premium reserves	437.9	498.6	584.9
Change in reinsurance recoverable	(60.0)	(179.1)	(248.1)
Change in expenses and taxes payable	67.5	11.0	4.6
Other, net	(24.7)	(14.7)	(31.7)
Net cash provided by operating activities	707.6	602.9	551.3
Cash Flows From Investing Activities
Proceeds from disposals and maturities of fixed maturities	1,317.5	1,336.9	1,094.2
Proceeds from disposals of equity securities and other investments	172.8	250.3	158.4
Purchase of fixed maturities	(1,829.1)	(1,588.3)	(1,425.6)
Purchase of equity securities and other investments	(255.1)	(332.2)	(178.3)
Capital expenditures	(14.9)	(13.3)	(13.1)
Net proceeds from sale of Chaucer business		34.7	635.7
Net cash provided by (used in) investing activities	(608.8)	(311.9)	271.3
Cash Flows From Financing Activities
Proceeds from exercise of employee stock options	6.3	14.4	14.8
Proceeds from debt borrowings, net	296.4
Change in cash collateral related to securities lending program		(5.0)	(19.0)
Dividends paid to shareholders	(99.5)	(386.2)	(94.3)
Repayment of debt	(175.8)	(151.1)	(11.6)
Repurchases of common stock	(212.8)	(563.6)	(57.7)
Other financing activities	(8.5)	(7.8)	(3.2)
Net cash used in financing activities	(193.9)	(1,099.3)	(171.0)
Effect of exchange rate changes on cash			(4.0)
Net change in cash and cash equivalents	(95.1)	(808.3)	647.6
Net change in cash related to discontinued operations		3.3	75.2
Cash and cash equivalents, beginning of year	215.7	1,020.7	297.9
Cash and cash equivalents, end of year	120.6	215.7	1,020.7
Supplemental Cash Flow Information
Interest payments	33.9	37.0	47.8
Income tax payments, net	97.0	113.7	57.0
Continuing Operations [Member]
Adjustments to reconcile net income to net cash provided by operating activities:
Net realized investment (gains) losses	$ (5.1)	$ (109.0)	$ 51.3