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Comprehensive Income - Changes in Each Component of Accumulated Other Comprehensive (Loss) Income (Parenthetical) (Detail) - USD ($) $ in Millions | 12 Months Ended
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Apr. 30, 2018 | Apr. 30, 2017 | Apr. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |
Tax effect on reclassifications of realized net losses | $ 0.8 | $ 1.2 | $ 1.1
Accumulated Defined Benefit Plan Adjustment |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |
Tax effects on unrealized gains (losses) | 2.5 | 1.9 | $ (2.3)
Unrealized (Losses) Gains on Interest Rate Swap |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |
Tax effects on unrealized gains (losses) | 0.8 | $ (0.4) |
Tax effect on reclassifications of realized net losses | $ 0.3 |  |

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- Definition Amount, before reclassification adjustment, of tax (expense) benefit for (increase) decrease in accumulated other comprehensive income for defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=84228862&loc=d3e640-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (i-k) -URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669646-108580 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 45 -Paragraph 19 -URI http://asc.fasb.org/extlink&oid=96868048&loc=SL4569616-111683 + Details
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- Definition Amount of tax expense (benefit) of reclassification adjustment from accumulated other comprehensive income of accumulated gain (loss) realized from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's deferred hedging gain (loss). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=84228862&loc=d3e640-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph c -URI http://asc.fasb.org/extlink&oid=84234895&loc=d3e41638-113959 + Details
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