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Goodwill and Other Intangible Assets - Goodwill Rollforward (Details) - USD ($) $ in Millions	2 Months Ended	3 Months Ended		5 Months Ended	9 Months Ended
	Jun. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017
Goodwill [Roll Forward]
Goodwill, beginning balance					$ 25.9
Unallocated acquisition date fair value of intangible assets, beginning balance		$ 436.2
Intangible assets, gross (including goodwill), beginning balance					96.9
Attribution of acquisition date fair value estimates between goodwill and other intangible assets		(436.2)
Amortization		7.4	$ 2.4		12.9	$ 7.3
Goodwill, ending balance		321.4		$ 321.4	321.4
Intangible assets, net (including goodwill), ending balance		485.2		485.2	485.2
Unallocated acquisition date fair value of intangible assets, ending balance	$ 436.2	0.0		0.0	0.0
Intangible assets, gross (including goodwill), ending balanc		532.9		532.9	532.9
Other Intangible Assets
Goodwill [Roll Forward]
Intangible assets, net (excluding goodwill), beginning balance		30.7	14.4		36.2	19.3
Attribution of acquisition date fair value estimates between goodwill and other intangible assets		140.9			140.9
Foreign currency translation		(0.4)			(0.4)
Amortization		(7.4)	(2.5)		(12.9)	(7.4)
Intangible assets, net (excluding goodwill), ending balance	30.7	163.8	11.9	163.8	163.8	11.9
Intangible Asset and Goodwill
Goodwill [Roll Forward]
Intangible assets, gross (including goodwill), beginning balance		492.8	40.3		62.1	45.2
Attribution of acquisition date fair value estimates between goodwill and other intangible assets					436.2
Foreign currency translation		(1.6)			(1.6)
Adjustments to reflect acquisition date fair value		1.4			1.4
Amortization		(7.4)	(2.5)		(12.9)	(7.4)
Intangible assets, gross (including goodwill), ending balanc	492.8	485.2	37.8	485.2	485.2	37.8
Goodwill
Goodwill [Roll Forward]
Goodwill, beginning balance		25.9	25.9		25.9	25.9
Attribution of acquisition date fair value estimates between goodwill and other intangible assets		295.3			295.3
Foreign currency translation		(1.2)			(1.2)
Adjustments to reflect acquisition date fair value		1.4			1.4
Goodwill, ending balance	25.9	321.4	$ 25.9	321.4	321.4	$ 25.9
NSM Holding Co LLC
Goodwill [Roll Forward]
Intangible assets, net (including goodwill), ending balance		431.0		431.0	$ 431.0
NSM Holding Co LLC \| Other Intangible Assets
Goodwill [Roll Forward]
Foreign currency translation				0.4
Amortization	$ 1.8	$ 3.2
NSM Holding Co LLC \| Goodwill
Goodwill [Roll Forward]
Foreign currency translation				$ 1.2