Accounting Developments (Details) - USD ($) $ in Millions	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019	Jan. 01, 2019
New Accounting Pronouncements or Change in Accounting Principle
Mortgage Loans, Carrying Amount	$ 2,434.4		$ 2,397.0
Reinsurance Recoverable	4,786.2		4,780.7
Accounts and Premiums Receivable	1,625.5		1,602.9
Deferred Income Tax	155.6		95.4
Other Liabilities	2,022.2		1,856.5
Retained Earnings	$ 11,019.2		10,728.7
Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle
Mortgage Loans, Carrying Amount		$ 2,388.7	2,397.0
Reinsurance Recoverable		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, Carrying Amount		(8.3)
Reinsurance Recoverable		(1.8)
Accounts and Premiums Receivable		(13.5)
Deferred Income Tax		(5.0)
Other Liabilities		0.3
Retained Earnings		$ (18.9)
Accounting Standards Update 2016-02 | Restatement Adjustment
New Accounting Pronouncements or Change in Accounting Principle
Retained Earnings				$ 3.4