Other Assets - Narrative (Details) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2017 USD ($) joint_venture	Dec. 31, 2016 USD ($)	Sep. 30, 2017 USD ($)
Other Assets [Abstract]
Number of joint ventures | joint_venture	7
Ownership percentage of joint ventures	50.00%
Sales to equity affiliates	$ 50	$ 67
Deferred profit generated from sales to equity affiliates	$ 1		$ 2
Estimated useful life of exchange and rental assets	15 years
Depreciation expense, exchange and rental assets	$ 3	$ 2