Significant Accounting Policies Schedule of New Accounting Pronouncements (Details) - USD ($) $ in Millions	Dec. 31, 2020	Jan. 01, 2020	Dec. 31, 2019
New Accounting Pronouncements or Change in Accounting Principle
Mortgage Loans	$ 2,432.1		$ 2,397.0
Reinsurance Recoverables	10,666.0		4,780.7
Accounts and Premiums Receivable	1,519.3		1,602.9
Deferred Income Tax	416.1		95.4
Other Liabilities	2,471.4		1,880.5
Retained Earnings	$ 11,269.6		10,728.7
Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle
Mortgage Loans		$ 2,388.7	2,397.0
Reinsurance Recoverables		4,778.9	4,780.7
Accounts and Premiums Receivable		1,589.4	1,602.9
Deferred Income Tax		90.4	95.4
Other Liabilities		1,856.8	1,856.5
Retained Earnings		10,709.8	$ 10,728.7
Accounting Standards Update 2016-13 | Restatement Adjustment
New Accounting Pronouncements or Change in Accounting Principle
Mortgage Loans		(8.3)
Reinsurance Recoverables		(1.8)
Accounts and Premiums Receivable		(13.5)
Deferred Income Tax		(5.0)
Other Liabilities		0.3
Retained Earnings		$ (18.9)