Insurance (Obligations Under Funding Agreements - Narrative) (Details) - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Insurance [Abstract]
Funding agreements issued to certain SPEs	$ 42.7	$ 39.7	$ 35.1
Funding agreements repaid to certain SPEs	41.4	38.5	$ 35.5
Outstanding funding agreements to certain SPEs	$ 34.2	$ 30.8