U.S. and Europe Retrospective Responsibility Plans - Preferred Stock Rollforward (Details) - USD ($)	3 Months Ended	6 Months Ended
	Mar. 31, 2017	Mar. 31, 2017
Preferred Stock and Right to Recover for Covered Losses [Roll Forward]
Recovery through conversion rate adjustment		$ 120,000,000
Right to recover for covered losses, beginning		34,000,000
VE territory covered losses incurred		(163,000,000)
Right to recover for covered losses, ending	$ 77,000,000	77,000,000
U.K.& I preferred stock
Preferred Stock and Right to Recover for Covered Losses [Roll Forward]
Preferred stock, beginning		2,516,000,000
Recovery through conversion rate adjustment	(119,000,000)	(119,000,000)
Preferred stock, ending	2,397,000,000	2,397,000,000
Europe preferred stock
Preferred Stock and Right to Recover for Covered Losses [Roll Forward]
Preferred stock, beginning		3,201,000,000
Recovery through conversion rate adjustment	(1,000,000)	(1,000,000)
Preferred stock, ending	$ 3,200,000,000	$ 3,200,000,000