Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended	6 Months Ended
Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016
Income Tax Contingency [Line Items]
Income tax expense	$ 3,000,000	$ 15,000,000	$ 20,000,000	$ 12,000,000
Statutory income tax rate	35.00%	35.00%	35.00%	35.00%
Income withholding taxes and foreign losses uncertain tax positions	$ 0	$ 0	$ 0	$ 10,000,000
Maximum
Income Tax Contingency [Line Items]
Decrease in gross unrecognized tax benefits	3,000,000
United Kingdom (“U.K.”) Tax Authority
Income Tax Contingency [Line Items]
Decrease in gross unrecognized tax benefits	$ 13,000,000

Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=65882285&loc=d3e32687-109319

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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-Section S99
-Paragraph 1
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-URI http://asc.fasb.org/extlink&oid=34349781&loc=d3e330036-122817

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
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-URI http://asc.fasb.org/extlink&oid=65882285&loc=d3e32698-109319

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Paragraph 9
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
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-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 3: http://www.xbrl.org/2003/role/presentationRef
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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to foreign income tax expense (benefit).

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
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-URI http://asc.fasb.org/extlink&oid=65882285&loc=d3e32687-109319

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Amount of decrease in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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