Schedule 2 - Parent Company Financial Statements - Statements of Income (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Net Investment Income	$ 82.6	$ 85.9	$ 77.1	$ 81.6	$ 79.9	$ 77.7	$ 73.7	$ 67.0	$ 327.2	$ 298.3	$ 302.6
Net Realized Gains (Losses) on Sales of Investments	11.5	8.1	26.4	10.5	9.1	11.6	5.6	6.8	56.5	33.1	52.1
Net Impairment Losses Recognized in Earnings	(3.8)	(2.9)	(2.6)	(5.0)	(8.7)	(8.3)	(6.4)	(9.3)	(14.3)	(32.7)	(27.2)
Total Revenues	697.3	690.3	684.4	651.4	642.7	640.7	627.2	611.3	2,723.4	2,521.9	2,340.8
Interest Expense									34.9	44.4	46.5
Loss from Early Extinguishment of Debt									0.0	0.0	9.1
Total Expenses	653.1	622.0	632.3	654.9	602.1	673.9	624.6	617.8	2,562.3	2,518.4	2,240.5
Income Tax Benefit	(8.3)	(20.5)	(15.5)	3.1	(11.5)	14.9	1.5	4.3	(41.2)	9.2	(20.1)
Net Income	$ 36.9	$ 47.7	$ 36.6	$ (0.3)	$ 31.2	$ (16.3)	$ 4.0	$ (2.1)	120.9	16.8	85.7
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Net Investment Income									3.0	2.0	1.7
Net Realized Gains (Losses) on Sales of Investments									0.6	0.1	(0.1)
Net Impairment Losses Recognized in Earnings									0.0	0.0	(1.6)
Total Revenues									3.6	2.1	0.0
Interest Expense									36.6	45.2	47.3
Loss from Early Extinguishment of Debt									0.0	0.0	9.1
Other Operating (Benefits) Expenses									(5.1)	(6.1)	17.6
Total Expenses									31.5	39.1	74.0
Loss before Income Taxes and Equity in Net Income of Subsidiaries									(27.9)	(37.0)	(74.0)
Income Tax Benefit									21.2	13.4	26.5
Loss before Equity in Net Income of Subsidiaries									(6.7)	(23.6)	(47.5)
Equity in Net Income of Subsidiaries									127.6	40.4	133.2
Net Income									$ 120.9	$ 16.8	$ 85.7