Liability for Unpaid Claims and CAE Liability for Unpaid Claims and Claim Adjustment Expenses (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense
Balance at January 1	$ 23,222.0	$ 23,249.5
Less Reinsurance Recoverable	2,182.0	2,163.6
Net Balance at January 1	21,040.0	21,085.9
Incurred Related to
Current Year	4,421.9	4,046.2
Total Incurred	5,093.3	4,889.4
Paid Related to
Current Year	(1,589.1)	(1,463.9)
Prior Years	(3,491.2)	(3,478.8)
Total Paid	(5,080.3)	(4,942.7)
Net Balance at September 30	21,053.0	21,032.6
Plus Reinsurance Recoverable	2,204.0	2,163.9
Balance at September 30	23,257.0	23,196.5
Amount Related to Interest
Incurred Related to
Prior Years	807.2	836.5
Incurred Claims
Incurred Related to
Prior Years	(65.6)	(143.3)
Foreign Currency
Incurred Related to
Prior Years	$ (70.2)	$ 150.0