Prior Period Consolidated Financial Statements - Consolidated Balance Sheet (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022		Dec. 31, 2021		Dec. 31, 2020
Assets
Total investments	$ 6,830.5	$ 6,587.6	[1]
Cash	29.7	42.8	[1],[2]	$ 133.7	[2]	$ 22.3	[2]
Deferred policy acquisition costs	336.3	330.6	[1]
Reinsurance balances receivable	480.5	468.0	[1]
Deposit asset on reinsurance	2,496.6	2,516.6	[1]
Intangible assets	170.3	185.2	[1]
Goodwill	54.3	54.3	[1]	43.5		43.5
Other assets	357.6	328.7	[1]
Separate account asset	3,294.1	2,792.3	[1]
Total assets	14,049.9	13,306.1	[1]	14,460.2
Policy liabilities
Investment contract and policy reserves		0.0
Future policy benefit reserves	1,761.8	1,718.0	[1]
Policyholders' account balances	5,187.0	5,260.6	[1]
Unpaid claims and claim expenses	581.7	564.0	[1]
Unearned premiums	300.9	266.1	[1]
Total policy liabilities		7,808.7
Other policyholder funds	916.0	809.3	[1]
Other liabilities	287.1	299.5	[1]
Short-term debt	0.0	249.0	[1]
Long-term debt	546.0	249.0	[1]
Separate Account variable annuity liabilities	3,294.1	2,792.3	[1]
Total liabilities	12,874.6	12,207.8	[1]
Preferred stock, $0.001 par value, authorized 1,000,000 shares; none issued	0.0	0.0	[1]
Common stock, $0.001 par value, authorized 75,000,000 shares; issued, 2023, 66,747,821; 2022, 66,618,465	0.1	0.1	[1]
Additional paid-in capital	510.9	502.6	[1]
Retained earnings	1,502.2	1,512.4	[1]
Accumulated other comprehensive income (loss), net of tax:
Net unrealized investment losses on fixed maturity securities	(328.3)	(449.6)	[1]
Net reserve remeasurements attributable to discount rates	21.9	59.0	[1]
Net funded status of benefit plans	(7.6)	(8.8)	[1]
Treasury stock, at cost, 2023, 25,911,087 shares; 2022, 25,714,153 shares	(523.9)	(517.4)	[1]
Total shareholders' equity	1,175.3	1,098.3	[3]	1,499.0	[3]
Total liabilities and shareholders' equity	$ 14,049.9	13,306.1	[1]
Previously Reported
Assets
Total investments		6,587.6
Cash		42.8		133.7		22.3
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 asset		2,792.3
Total assets		13,446.8
Policy liabilities
Investment contract and policy reserves		6,968.0
Future policy benefit reserves		0.0
Policyholders' account balances		0.0
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, $0.001 par value, authorized 1,000,000 shares; none issued		0.0
Common stock, $0.001 par value, authorized 75,000,000 shares; issued, 2023, 66,747,821; 2022, 66,618,465		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, 2023, 25,911,087 shares; 2022, 25,714,153 shares		(517.4)
Total shareholders' equity		1,088.2		1,807.4
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 asset		0.0
Total assets		(140.7)
Policy liabilities
Investment contract and policy reserves		(151.9)
Future policy benefit reserves		0.0
Policyholders' account balances		0.0
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, $0.001 par value, authorized 1,000,000 shares; none issued		0.0
Common stock, $0.001 par value, authorized 75,000,000 shares; issued, 2023, 66,747,821; 2022, 66,618,465		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, 2023, 25,911,087 shares; 2022, 25,714,153 shares		0.0
Total shareholders' equity		10.1		$ (308.4)
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 asset		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, $0.001 par value, authorized 1,000,000 shares; none issued		0.0
Common stock, $0.001 par value, authorized 75,000,000 shares; issued, 2023, 66,747,821; 2022, 66,618,465		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, 2023, 25,911,087 shares; 2022, 25,714,153 shares		0.0
Total shareholders' equity		0.0
Total liabilities and shareholders' equity		$ 0.0

[1] Recast for the adoption of ASU 2018-12. See Note 1 of the Consolidated Financial Statements

[2] Recast for the adoption of ASU 2018-12. See Note 1 of the Consolidated Financial Statements

[3] Recast for the adoption of ASU 2018-12. See Note 1 of the Consolidated Financial Statements