Reclassifications out of Accumulated Other Comprehensive Loss - Summary of Reclassifications out of Accumulated Other Comprehensive Loss (Detail) $ in Millions	6 Months Ended
Jun. 30, 2018 USD ($)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications	$ (0.4)
Defined Benefit Pension Plan Items [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications	(0.4)
Reclassification Out of Accumulated Other Comprehensive Income/(Loss) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	0.4
Income tax expense	0.0
Total reclassifications	0.4
Reclassification Out of Accumulated Other Comprehensive Income/(Loss) [Member] \| Defined Benefit Pension Plan Items [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of prior service credit	(0.6)
Amortization of actuarial net losses	$ 1.0

Amount, before tax, of reclassification adjustment from accumulated other comprehensive (income) loss for prior service cost (credit) of defined benefit plan.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount, before tax, of reclassification adjustment from accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan.

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Amount before tax of reclassification adjustments of other comprehensive income (loss).

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Amount after tax of reclassification adjustments of other comprehensive income (loss).

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Amount of tax expense (benefit) of reclassification adjustment from accumulated other comprehensive income (loss).

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