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Debt - URS Senior Notes (Details) - USD ($) $ in Millions | Oct. 24, 2014 | Mar. 31, 2018 | Sep. 30, 2017 | Apr. 03, 2017 | Oct. 17, 2014
--- | --- | --- | --- | --- | ---
Debt |  |  |  |  |
Debt |  | $ 3,999.1 | $ 3,896.4 |  |
URS Senior Notes |  |  |  |  |
Debt |  |  |  |  |
Debt |  | 247.8 | $ 247.7 |  | $ 1,000.0
URS Senior Notes | 2017 URS Senor Notes |  |  |  |  |
Debt |  |  |  |  |
Interest rate (as a percent) |  |  |  |  | 3.85%
Redemption price equal to principal using proceeds from equity offerings (in percent) | 101.00% |  |  |  |
Amount of debt redeemed | $ 572.3 |  |  | $ 179.2 |
URS Senior Notes | 2022 URS Senior Notes |  |  |  |  |
Debt |  |  |  |  |
Interest rate (as a percent) |  |  |  |  | 5.00%
Debt |  | 247.8 |  |  |
Fair value of debt instrument |  | $ 253.5 |  |  |

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