Long-Term Debt - Summary of Debt Refinancing Costs (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Gross cost at January 1	$ 1.5	$ 2.7
Capitalized in the year	0.3	1.5
Written down in the year	0.0	(2.7)
Gross cost	1.8	1.5	$ 2.7
Accumulated amortization,Beginning Balance	(0.1)	(1.8)
Amortization in the year	(0.4)	(1.0)	(0.7)
Amortization written down in the year	0.0	2.7
Accumulated amortization,Ending balance	(0.5)	(0.1)	$ (1.8)
Deferred finance costs, net	$ 1.3	$ 1.4

Amount of amortization expense attributable to debt issuance costs.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(8))
-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28555-108399

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration