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Income Taxes - Additional Disclosures (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended	12 Months Ended
	Oct. 31, 2014	Jun. 30, 2016	Sep. 30, 2016	Sep. 30, 2015
Deferred tax assets:
Compensation and benefit accruals not currently deductible			$ 150.5	$ 166.7
Net operating loss carryforwards			378.0	195.9
Self insurance reserves			24.7	46.8
Research and experimentation and other tax credits			125.3	43.0
Pension liability			180.9	165.6
Accrued liabilities			221.0	267.3
Other			4.3	11.4
Total deferred tax assets			1,084.7	896.7
Deferred tax liabilities:
Unearned revenue			(212.6)	(101.9)
Depreciation and amortization			(113.0)	(76.5)
Acquired intangible assets			(155.5)	(219.2)
Investment in subsidiaries			(261.4)	(239.2)
Total deferred tax liabilities			(742.5)	(636.8)
Valuation allowance			(183.8)	(239.4)
Net deferred tax assets			158.4	20.5
Potential increase to equity for deferred tax assets related to equity compensation			3.8
Income Taxes, additional disclosures
Net deferred tax asset likely to be realized and for which no additional valuation allowance has been provided			900.9
Change in valuation allowance			(55.6)
Foreign net operating loss carryforwards			10.7
Tax-deductible goodwill			210.8	261.2
Gross book-tax differences from non-U.S. subsidiaries			1,600.0
Deferred tax liability of foreign subsidiaries			113.2
Liability for unrecognized tax benefits, including potential interest and penalties, net of related tax benefit			96.8	107.6
Unrecognized tax benefits that would be included in the effective tax rate if recognized				71.6
Reconciliation of the beginning and ending amount of gross unrecognized tax benefits
Balance at the beginning of the year	$ 47.5		95.2	47.5
Gross increase due to acquisitions				49.4
Gross increase in current period's tax positions			7.6	6.0
Gross increase in prior years' tax positions			5.2	6.4
Gross decrease in prior years' tax positions			(16.6)	(0.2)
Decrease due to settlement with tax authorities			(3.2)	(2.0)
Decrease due to lapse of statute of limitations			(1.8)	(4.6)
Gross change due to foreign exchange fluctuations			1.5	(7.3)
Balance at the end of the year			87.9	95.2
Accrued interest, excluding any related income tax benefits			12.5	13.9
Accrued penalties, excluding any related income tax benefits			2.6	$ 3.5
United Kingdom and Australia
Income Taxes, additional disclosures
Change in valuation allowance			(36.2)
United Kingdom
Income Taxes, additional disclosures
Change in valuation allowance			(23.3)
Decrease in valuation allowance due to change in tax rate			(27.2)
Australia
Income Taxes, additional disclosures
Change in valuation allowance		$ (12.9)
URS Corporation | United Kingdom
Income Taxes, additional disclosures
Change in valuation allowance	92.8
URS Corporation | Australia
Income Taxes, additional disclosures
Change in valuation allowance	2.9
URS Corporation | Canada
Income Taxes, additional disclosures
Change in valuation allowance	22.5
URS Corporation | United States
Income Taxes, additional disclosures
Change in valuation allowance	$ 9.3
California Franchise Tax Board | Enterprise Zone Tax Credits
Income Taxes, additional disclosures
Unused federal and state tax credits			6.2
Federal
Income Taxes, additional disclosures
Net operating loss carryforwards			503.8
Federal | Research and development
Income Taxes, additional disclosures
Unused federal and state tax credits			67.4
State
Income Taxes, additional disclosures
Net operating loss carryforwards			960.1
State | Research and development
Income Taxes, additional disclosures
Unused federal and state tax credits			19.2
Foreign
Income Taxes, additional disclosures
Net operating loss carryforwards			$ 789.6

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