Schedule 2 - Parent Company Financial Statements - Statements of Income (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Financial Statements, Captions [Line Items]
Net Investment Income	$ 91.3	$ 92.0	$ 78.4	$ 79.2	$ 82.6	$ 85.9	$ 77.1	$ 81.6	$ 340.9	$ 327.2	$ 298.3
Equity Securities, FV-NI, Gain (Loss)	(76.4)	11.0	0.4	0.7					(64.3)	0.0	0.0
Net Realized Gains (Losses) on Sales of Investments									26.4	56.5	33.1
Debt and Equity Securities, Realized Gain (Loss)	16.4	3.6	3.8	2.6	11.5	8.1	26.4	10.5	26.4	56.5	33.1
Net Impairment Losses Recognized in Earnings	(2.2)	(1.8)	0.0	(0.5)	(3.8)	(2.9)	(2.6)	(5.0)	(4.5)	(14.3)	(32.7)
Total Revenues	1,094.7	1,195.5	741.9	693.0	697.3	690.3	684.4	651.4	3,725.1	2,723.4	2,521.9
Interest Expense									43.3	34.9	44.4
Total Expenses	1,088.6	1,115.0	696.4	626.0	653.1	622.0	632.3	654.9	3,526.0	2,562.3	2,518.4
Income Tax Benefit	(1.1)	11.8	(8.0)	(13.4)	(8.3)	(20.5)	(15.5)	3.1	(10.7)	(41.2)	9.2
Net Income	$ 6.5	$ 92.2	$ 37.6	$ 53.8	$ 36.9	$ 47.7	$ 36.6	$ (0.3)	190.1	120.9	16.8
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Net Investment Income									2.5	3.0	2.0
Equity Securities, FV-NI, Gain (Loss)									1.4	0.0	0.0
Debt and Equity Securities, Realized Gain (Loss)									(0.7)	0.6	0.1
Total Revenues									3.2	3.6	2.1
Interest Expense									37.6	36.6	45.2
Other Operating (Benefits) Expenses									26.3	(5.1)	(6.1)
Total Expenses									63.9	31.5	39.1
Loss before Income Taxes and Equity in Net Income of Subsidiaries									(60.7)	(27.9)	(37.0)
Income Tax Benefit									12.2	21.2	13.4
Loss before Equity in Net Income of Subsidiaries									(48.5)	(6.7)	(23.6)
Equity in Net Income of Subsidiaries									238.6	127.6	40.4
Net Income									$ 190.1	$ 120.9	$ 16.8