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Financial Instruments - Schedule of Financial Instruments (Details) - USD ($) $ in Millions | Sep. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | --- | ---
Derivative [Line Items] |  |  |
Interest rate cap, fair value | $ 22.2 |  | $ 5.4
Cap agreement effective 1/17/17-12/31/18 |  |  |
Derivative [Line Items] |  |  |
Notional amount | 1,400.0 |  |
Interest rate cap, fair value | 3.2 |  | 5.4
Cap agreement effective 12/31/18-12/31/20 |  |  |
Derivative [Line Items] |  |  |
Notional amount | 1,400.0 | $ 1,400.0 |
Interest rate cap, fair value | $ 19.0 |  | $ 0.0

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