Prior Period Consolidated Financial Statements - Consolidated Balance Sheet (Unaudited) (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Dec. 31, 2021
Assets
Total investments	$ 6,519.4	$ 6,587.6
Cash	34.5	42.8	$ 36.2	$ 133.7
Deferred policy acquisition costs	334.9	330.6
Reinsurance balances receivable	441.1	468.0
Deposit asset on reinsurance	2,515.7	2,516.6
Intangible assets	174.0	185.2
Goodwill	54.3	54.3
Other assets	364.6	328.7
Separate Account variable annuity assets	2,974.0	2,792.3
Total assets	13,412.5	13,306.1
Policy liabilities
Investment contract and policy reserves		0.0
Future policy benefit reserves	1,613.4	1,718.0
Policyholders' account balances	5,204.0	5,260.6
Unpaid claims and claim expenses	586.6	564.0
Unearned premiums	296.4	266.1
Total policy liabilities	7,700.4	7,808.7
Other policyholder funds	915.9	809.3
Other liabilities	224.8	299.5
Short-term debt	0.0	249.0
Long-term debt	546.1	249.0
Separate Account variable annuity liabilities	2,974.0	2,792.3
Total liabilities	12,361.2	12,207.8
Preferred stock	0.0	0.0
Common stock	0.1	0.1
Additional paid-in capital	508.5	502.6
Retained earnings	1,476.5	1,512.4
Accumulated other comprehensive income (loss), net of tax:
Net unrealized investment losses on fixed maturity securities	(534.7)	(449.6)
Net reserve remeasurements attributable to discount rates	133.6	59.0
Net funded status of benefit plans	(8.8)	(8.8)
Treasury stock, at cost	(523.9)	(517.4)
Total shareholders’ equity	1,051.3	1,098.3	1,092.3
Total liabilities and shareholders’ equity	$ 13,412.5	13,306.1
Previously Reported
Assets
Total investments		6,587.6
Cash		42.8	36.2	133.7
Deferred policy acquisition costs		433.1
Reinsurance balances receivable		506.2
Deposit asset on reinsurance		2,516.6
Intangible assets		185.2
Goodwill		54.3
Other assets		328.7
Separate Account variable annuity assets		2,792.3
Total assets		13,446.8
Policy liabilities
Investment contract and policy reserves		6,968.0
Future policy benefit reserves
Unpaid claims and claim expenses		585.1
Unearned premiums		264.2
Total policy liabilities		7,817.3
Other policyholder funds		954.0
Other liabilities		297.0
Short-term debt		249.0
Long-term debt		249.0
Separate Account variable annuity liabilities		2,792.3
Total liabilities		12,358.6
Preferred stock		0.0
Common stock		0.1
Additional paid-in capital		502.6
Retained earnings		1,468.6
Accumulated other comprehensive income (loss), net of tax:
Net unrealized investment losses on fixed maturity securities		(356.9)
Net reserve remeasurements attributable to discount rates		0.0
Net funded status of benefit plans		(8.8)
Treasury stock, at cost		(517.4)
Total shareholders’ equity		1,088.2	1,076.6
Total liabilities and shareholders’ equity		13,446.8
Effect of the Adoption of ASU 2018-12
Assets
Total investments		0.0
Cash		0.0	0.0	$ 0.0
Deferred policy acquisition costs		(102.5)
Reinsurance balances receivable		(38.2)
Deposit asset on reinsurance		0.0
Intangible assets		0.0
Goodwill		0.0
Other assets		0.0
Separate Account variable annuity assets		0.0
Total assets		(140.7)
Policy liabilities
Investment contract and policy reserves		(151.9)
Future policy benefit reserves
Unpaid claims and claim expenses		(2.9)
Unearned premiums		1.9
Total policy liabilities		(152.9)
Other policyholder funds		(0.4)
Other liabilities		2.5
Short-term debt		0.0
Long-term debt		0.0
Separate Account variable annuity liabilities		0.0
Total liabilities		(150.8)
Preferred stock		0.0
Common stock		0.0
Additional paid-in capital		0.0
Retained earnings		43.8
Accumulated other comprehensive income (loss), net of tax:
Net unrealized investment losses on fixed maturity securities		(92.7)
Net reserve remeasurements attributable to discount rates		59.0
Net funded status of benefit plans		0.0
Treasury stock, at cost		0.0
Total shareholders’ equity		10.1	$ 15.7
Total liabilities and shareholders’ equity		(140.7)
Reclassifications
Assets
Total investments		0.0
Cash		0.0
Deferred policy acquisition costs		0.0
Reinsurance balances receivable		0.0
Deposit asset on reinsurance		0.0
Intangible assets		0.0
Goodwill		0.0
Other assets		0.0
Separate Account variable annuity assets		0.0
Total assets		0.0
Policy liabilities
Investment contract and policy reserves		(6,816.1)
Future policy benefit reserves		1,718.0
Policyholders' account balances		5,260.6
Unpaid claims and claim expenses		(18.2)
Unearned premiums		0.0
Total policy liabilities		144.3
Other policyholder funds		(144.3)
Other liabilities		0.0
Short-term debt		0.0
Long-term debt		0.0
Separate Account variable annuity liabilities		0.0
Total liabilities		0.0
Preferred stock		0.0
Common stock		0.0
Additional paid-in capital		0.0
Retained earnings		0.0
Accumulated other comprehensive income (loss), net of tax:
Net unrealized investment losses on fixed maturity securities		0.0
Net reserve remeasurements attributable to discount rates		0.0
Net funded status of benefit plans		0.0
Treasury stock, at cost		0.0
Total shareholders’ equity		0.0
Total liabilities and shareholders’ equity		$ 0.0