Segment Information - Long-Lived Assets and Revenue by Geographical Areas (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Segment Reporting Information [Line Items]
Revenues	$ 841	$ 811	$ 745	$ 849	$ 750	$ 710	$ 677	$ 829	$ 3,246	$ 2,966	$ 3,027
Long-lived Assets	203				220				203	220
United States
Segment Reporting Information [Line Items]
Revenues									1,360	1,171	1,141
Long-lived Assets	138				149				138	149
United Kingdom
Segment Reporting Information [Line Items]
Revenues									491	477	467
Long-lived Assets	23				30				23	30
All Other Territories
Segment Reporting Information [Line Items]
Revenues									1,395	1,318	$ 1,419
Long-lived Assets	$ 42				$ 41				$ 42	$ 41