Schedule II - Condensed Financial Information of Registrant Statements of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Cash flows from operating activities:
Net income	$ 190.6	$ 52.6	$ 155.5	$ 199.8	$ 512.1	$ (91.7)	$ (40.1)	$ 306.3	$ 598.6	$ 686.4	$ 1,534.3
Adjustments to reconcile net income to net cash provided by operating activities:
Federal income taxes									47.1	54.7	(151.4)
Realized Investment Gains (Losses)									(67.0)	62.2	6.9
Other - net									136.3	1.9	160.9
Total									880.4	1,170.6	1,311.7
Cash flows from investing activities:
Maturities and early calls									1,353.2	1,356.1	1,410.9
Net decrease (increase) in short-term investments									(362.6)	(295.7)	183.9
Total									25.3	(415.0)	(936.5)
Cash flows from financing activities:
Issuance of common shares									31.1	26.6	60.0
Redemption of debentures and notes									(5.3)	0.0	(21.7)
Dividends on common shares									(275.5)	(579.7)	(1,019.2)
Treasury stock acquired									(535.3)	(281.2)	0.0
Other - net									1.8	1.5	2.5
Total									(783.2)	(832.7)	(335.7)
Increase (decrease) in cash									122.5	(77.1)	39.4
Cash, beginning of year				81.0					81.0
Cash, end of year	202.8				81.0				202.8	81.0
Parent Company [Member]
Cash flows from operating activities:
Net income									598.6	686.4	1,534.3
Adjustments to reconcile net income to net cash provided by operating activities:
Accounts receivable									(0.9)	0.0	0.6
Federal income taxes									(49.8)	59.4	(17.9)
Excess of equity in net (income) loss of subsidiaries over cash dividends received									68.6	(50.5)	(943.8)
Realized Investment Gains (Losses)									4.5	0.0	0.0
Other - net									14.7	(34.8)	11.5
Total									635.8	660.6	584.7
Cash flows from investing activities:
Sale of fixed assets for Company use									0.0	0.0	0.1
Purchase of fixed assets for Company use									(3.2)	(4.1)	(6.9)
Net repayment (issuance) of notes to related parties									54.6	64.9	(351.6)
Net decrease (increase) in short-term investments									13.9	(23.8)	21.5
Investment In,And Indebtedness Of Related Parties, Net									85.0	140.0	100.0
Total									150.3	176.9	(236.9)
Cash flows from financing activities:
Proceeds from Issuance of Long-term Debt									0.0	0.0	642.5
Net receipt (repayment) of notes and loans from related parties									(5.1)	(4.8)	(13.5)
Issuance of common shares									31.1	26.6	60.0
Redemption of debentures and notes									0.0	0.0	(19.5)
Dividends on common shares									(275.5)	(579.7)	(1,019.2)
Treasury stock acquired									(535.3)	(281.2)	0.0
Other - net									1.7	1.5	2.0
Total									(783.1)	(837.6)	(347.6)
Increase (decrease) in cash									3.0	0.0	0.1
Cash, beginning of year				$ 2.0				$ 2.0	2.0	2.0	1.9
Cash, end of year	$ 5.0				$ 2.0				$ 5.0	$ 2.0	$ 2.0