Schedule 2 - Parent Company Financial Statements - Statements of Income (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Condensed Income Statements, Captions [Line Items]
Net Investment Income	$ 93.9	$ 91.7	$ 96.0	$ 82.7	$ 91.3	$ 92.0	$ 78.4	$ 79.2	$ 364.3	$ 340.9	$ 327.2
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	39.2	9.8	25.5	64.4	(76.4)	11.0	0.4	0.7	138.9	(64.3)	0.0
Net Realized Gains on Sales of Investments	2.8	1.7	21.3	16.1	16.4	3.6	3.8	2.6	41.9	26.4	56.5
Net Impairment Losses Recognized in Earnings	(1.7)	(1.8)	(6.7)	(3.6)	(2.2)	(1.8)	0.0	(0.5)	(13.8)	(4.5)	(14.3)
Total Revenues	1,283.7	1,243.8	1,275.4	1,236.3	1,094.7	1,195.5	741.9	693.0	5,039.2	3,725.1	2,723.4
Interest Expense									42.5	43.3	34.9
Loss from Early Extinguishment of Debt	0.0	5.8	0.0	0.0					5.8	0.0	0.0
Total Expenses	1,126.8	1,082.3	1,126.9	1,041.6	1,088.6	1,115.0	696.4	626.0	4,377.6	3,526.0	2,562.3
Income Tax Benefit	(32.2)	(32.5)	(26.4)	(39.4)	(1.1)	11.8	(8.0)	(13.4)	(130.5)	(10.7)	(41.2)
Equity in Net Income of Subsidiaries									1.0	11.0	24.8
Net Income	$ 124.7	$ 129.0	$ 122.1	$ 155.3	$ 6.5	$ 92.2	$ 37.6	$ 53.8	531.1	190.1	120.9
Parent Company
Condensed Income Statements, Captions [Line Items]
Net Investment Income									2.1	2.5	3.0
Income (Loss) from Change in Fair Value of Equity and Convertible Securities									1.6	1.4	0.0
Net Realized Gains on Sales of Investments									0.3	(0.7)	0.6
Total Revenues									4.0	3.2	3.6
Interest Expense									28.5	37.6	36.6
Loss from Early Extinguishment of Debt									5.8	0.0	0.0
Other Operating (Benefits) Expenses									4.0	26.3	(5.1)
Total Expenses									38.3	63.9	31.5
Loss before Income Taxes and Equity in Net Income of Subsidiaries									(34.3)	(60.7)	(27.9)
Income Tax Benefit									9.4	12.2	21.2
Loss before Equity in Net Income of Subsidiaries									(24.9)	(48.5)	(6.7)
Equity in Net Income of Subsidiaries									556.0	238.6	127.6
Net Income									$ 531.1	$ 190.1	$ 120.9