Liability for Unpaid Claims and CAE Reconciliation of Policy and Contract Benefits and Reserves for Future Policy and Contract Benefits (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Insurance [Abstract]
Policy and Contract Benefits	$ 1,623.1	$ 1,605.2	$ 1,544.8
Reserves for Future Policy and Contract Benefits	44,668.5	45,601.6	44,502.2
Total Policy and Contract Benefits and Reserves for Future Policy and Contract Benefits	46,291.6		46,047.0
Life Reserves for Future Policy and Contract Benefits	8,278.7		8,105.7
Accident and Health Active Life Reserves	10,774.2		10,271.7
Adjustment Related to Unrealized Investment Gains and Losses	4,030.1		4,467.3
Liability for Unpaid Claims and Claim Adjustment Expense	$ 23,208.6	$ 23,222.0	$ 23,202.3	$ 23,249.5