Closed Block - Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Closed Block liabilities and assets designated to the Closed Block
Closed Block liabilities: Policyholder dividends obligation	$ 298.2	$ 202.7
Closed Block liabilities
Closed Block liabilities: Future policy benefits and claims	3,423.2	3,563.1
Closed Block liabilities: Other policyholder funds	6.0	6.6
Closed Block liabilities: Policyholder dividends payable	189.0	199.1
Closed Block liabilities: Policyholder dividends obligation	298.2	202.7
Closed Block liabilities: Other liabilities	8.7	7.8
Total Closed Block liabilities	3,925.1	3,979.3
Assets designated to the Closed Block
Closed Block assets: Fixed maturities, available-for-sale	2,353.3	2,269.6
Closed Block assets: Fixed maturities, trading	2.6	2.6
Closed Block assets: Equity securities	1.1	1.1
Closed Block assets: Mortgage loans	565.9	622.8
Closed Block assets: Policy loans	456.8	486.0
Closed Block assets: Other investments	61.7	46.4
Closed Block assets: Total investments	3,441.4	3,428.5
Closed Block assets: Cash and cash equivalents	23.2	47.5
Closed Block assets: Accrued investment income	35.4	38.1
Closed Block assets: Premiums due and other receivables	8.3	9.7
Closed Block assets: Deferred tax asset	24.2	29.5
Total assets designated to the Closed Block	3,532.5	3,553.3
Excess of Closed Block liabilities over assets designated to the Closed Block	392.6	426.0
Amounts included in accumulated other comprehensive income (loss)	0.9	0.9
Maximum future earnings to be recognized from Closed Block assets and liabilities	$ 393.5	$ 426.9	$ 476.8	$ 532.1