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Derivatives and Hedging Activity - Summary of Amounts of Derivative Gains (Losses) Recognized In Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Derivative gains (losses) recognized in accumulated other comprehensive loss | $ 15.0 | $ (11.5) | $ (2.8)
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion) | (5.2) | (8.2) | 0.7
Interest Rate Contracts [Member] |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Derivative gains (losses) recognized in accumulated other comprehensive loss | (0.9) | 12.4 |
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion) | 0.4 | 0.1 |
Foreign Exchange Contracts [Member] |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Derivative gains (losses) recognized in accumulated other comprehensive loss | 15.9 | (23.9) | (2.8)
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion) | (5.6) | (8.3) | 0.7
Commission Revenue [Member] |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Derivative gains (losses) recognized in accumulated other comprehensive loss | 10.4 | (24.0) | (3.3)
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion) | (8.7) | (9.1) | 0.7
Commission Revenue [Member] | Foreign Exchange Contracts [Member] |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Derivative gains (losses) recognized in accumulated other comprehensive loss | 10.4 | (24.0) | (3.3)
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion) | (8.7) | (9.1) | 0.7
Compensation Expense [Member] |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Derivative gains (losses) recognized in accumulated other comprehensive loss | 3.2 | 0.1 | 0.3
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion) | 1.8 | 0.5 |
Compensation Expense [Member] | Foreign Exchange Contracts [Member] |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Derivative gains (losses) recognized in accumulated other comprehensive loss | 3.2 | 0.1 | 0.3
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion) | 1.8 | 0.5 |
Operating Expense [Member] |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Derivative gains (losses) recognized in accumulated other comprehensive loss | 2.3 |  | 0.2
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion) | 1.3 | 0.3 |
Operating Expense [Member] | Foreign Exchange Contracts [Member] |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Derivative gains (losses) recognized in accumulated other comprehensive loss | 2.3 |  | $ 0.2
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion) | 1.3 | 0.3 |
Interest Expense [Member] |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Derivative gains (losses) recognized in accumulated other comprehensive loss | (0.9) | 12.4 |
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion) | 0.4 | 0.1 |
Interest Expense [Member] | Interest Rate Contracts [Member] |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Derivative gains (losses) recognized in accumulated other comprehensive loss | (0.9) | 12.4 |
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion) | $ 0.4 | $ 0.1 |

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- Definition The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4D -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624177-113959 + Details
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- Definition The effective portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments that was recognized in other comprehensive income during the current period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959 + Details
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