Closed Block - Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Closed Block liabilities and assets designated to the Closed Block
Closed Block liabilities: Policyholder dividend obligation related to earnings		$ 8.2
Closed Block liabilities: Policyholder dividend obligation related to net unrealized gains	$ 161.7	117.9
Closed Block liabilities
Closed Block liabilities: Future policy benefits and claims	3,899.7	4,068.4
Closed Block liabilities: Other policyholder funds	7.0	8.1
Closed Block liabilities: Policyholder dividends payable	219.6	232.5
Closed Block liabilities: Policyholder dividends obligation	161.7	126.1
Closed Block liabilities: Other liabilities	6.9	7.7
Total Closed Block liabilities	4,294.9	4,442.8
Assets designated to the Closed Block
Closed Block assets: Fixed maturities, available-for-sale	2,304.4	2,218.9
Closed Block assets: Fixed maturities, trading	2.9	6.7
Closed Block assets: Equity securities, available-for-sale	1.3	3.0
Closed Block assets: Mortgage loans	766.3	842.2
Closed Block assets: Policy loans	537.1	566.7
Closed Block assets: Other investments	49.4	62.4
Closed Block assets: Total investments	3,661.4	3,699.9
Closed Block assets: Cash and cash equivalents	13.2	76.2
Closed Block assets: Accrued investment income	41.1	43.6
Closed Block assets: Premiums due and other receivables	14.4	11.7
Closed Block assets: Deferred tax asset	34.6	62.3
Total assets designated to the Closed Block	3,764.7	3,893.7
Excess of Closed Block liabilities over assets designated to the Closed Block	530.2	549.1
Amounts included in accumulated other comprehensive income (loss)	1.9	0.7
Maximum future earnings to be recognized from Closed Block assets and liabilities	$ 532.1	$ 549.8	$ 596.4	$ 648.9