Organization - Use of the Proceeds from the IP (Detail) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017
Consolidation, Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Gross proceeds	$ 631.3
Underwriters’ discounts and commissions	29.2
IPO-related expenses	11.8
Redemption of 11% Senior Notes due 2021 (Note 8)	1,132.3	$ 25.9
Sponsor management termination fee	11.3
Debt refinancing expenses	11.4
Cash to balance sheet	68.6
Senior Notes [Member] \| 11% Senior Note [Member]
Consolidation, Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Redemption of 11% Senior Notes due 2021 (Note 8)	475.0
Call premium on redemption of 11% Senior Notes due 2021	13.1
Interest on redemption of 11% Senior Notes due 2021	$ 10.9

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