Reinsurance (Effect of Reinsurance on Net Premiums and Claims and Other Policy Benefits) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Premiums Earned, Net [Abstract]
Direct Premiums Earned	$ 76.1	$ 62.5	$ 61.6
Assumed Premiums Earned	12,149.7	11,341.3	10,642.4
Ceded Premiums Earned	(928.6)	(860.0)	(862.9)
Net premiums	11,297.2	10,543.8	9,841.1
Policyholder Benefits and Claims Incurred, Net [Abstract]
Policyholder Benefits and Claims Incurred, Direct	113.0	107.0	104.0
Policyholder Benefits and Claims Incurred, Assumed	11,404.0	9,997.0	9,282.0
Policyholder Benefits and Claims Incurred, Ceded	(1,320.0)	(785.0)	(867.0)
Claims and other policy benefits	$ 10,197.0	$ 9,319.0	$ 8,519.0