Basic and Diluted Earnings Per Share - Additional Information (Detail) - shares shares in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Common stock equivalents included in the computation of diluted shares outstanding	5	5	5	5
Common stock equivalents excluded from computation of average dilutive shares outstanding	3	1	3	1
Class B common stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Weighted-average as-converted common stock used in income allocation	405	405	405	405
Class C common stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Weighted-average as-converted common stock used in income allocation	58	77	63	78
U.K.& I preferred stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Weighted-average as-converted common stock used in income allocation	34
Europe preferred stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Weighted-average as-converted common stock used in income allocation	44		44