U.S. and Europe Retrospective Responsibility Plans (Details) $ / shares in Units, € in Millions, shares in Millions	3 Months Ended	6 Months Ended
	Mar. 31, 2017 USD ($) $ / shares shares	Mar. 31, 2017 EUR (€)	Mar. 31, 2017 USD ($) $ / shares shares	Sep. 30, 2016 USD ($) shares
Restricted Cash and Cash Equivalents Items [Line Items]
Escrow account	$ 1,029,000,000		$ 1,029,000,000	$ 1,027,000,000
VE territory covered loss, maximum amount of loss to allow adjustment of conversion rate during six-month period | €		€ 20
Right to recover for covered losses	$ 77,000,000		77,000,000	$ 34,000,000
Recovery through conversion rate adjustment			$ (120,000,000)
Preferred stock, conversion ratio				13.952
Closing stock price | $ / shares	$ 88.87		$ 88.87
U.K.& I preferred stock
Restricted Cash and Cash Equivalents Items [Line Items]
Recovery through conversion rate adjustment	$ 119,000,000		$ 119,000,000
Preferred stock, conversion ratio	13.3880		13.3880
Preferred stock, shares outstanding | shares	2		2	2
Europe preferred stock
Restricted Cash and Cash Equivalents Items [Line Items]
Recovery through conversion rate adjustment	$ 1,000,000		$ 1,000,000
Preferred stock, conversion ratio	13.9480		13.9480
Preferred stock, shares outstanding | shares	3		3	3