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Derivative Financial Instruments - Additional Information (Details) - Dec. 31, 2018 £ in Millions, $ in Millions | USD ($) derivative $ / unit | GBP (£) derivative $ / unit
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Derivative [Line Items] |  |
Number of contracts with a contingent feature in a net liability position | derivative | 0 | 0
Forward contracts |  |
Derivative [Line Items] |  |
Derivative, number of contracts | derivative | 2 | 2
Cash collateral held from counterparty | $ | $ 3.1 |
Cash collateral held at counterparty | $ | $ 28.0 |
Collar contracts |  |
Derivative [Line Items] |  |
Derivative, number of contracts | derivative | 2 | 2
Designated as Hedging Instrument | Net Investment Hedging | Forward contract expiring 2024 |  |
Derivative [Line Items] |  |
Derivative, notional amount | $ 400.0 | £ 285.8
Designated as Hedging Instrument | Net Investment Hedging | Forward contract expiring 2021 |  |
Derivative [Line Items] |  |
Derivative, notional amount | $ 450.0 | £ 325.3
Designated as Hedging Instrument | Net Investment Hedging | Collar contract-call option | Minimum |  |
Derivative [Line Items] |  |
Derivative, option strike price (in dollars per unit) | 1.288 | 1.288
Designated as Hedging Instrument | Net Investment Hedging | Collar contract-call option | Maximum |  |
Derivative [Line Items] |  |
Derivative, option strike price (in dollars per unit) | 1.535 | 1.535
Designated as Hedging Instrument | Net Investment Hedging | Collar contract-put option | Minimum |  |
Derivative [Line Items] |  |
Derivative, option strike price (in dollars per unit) | 1.318 | 1.318
Designated as Hedging Instrument | Net Investment Hedging | Collar contract-put option | Maximum |  |
Derivative [Line Items] |  |
Derivative, option strike price (in dollars per unit) | 1.448 | 1.448

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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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- Definition Amount of obligation to return cash collateral under master netting arrangements that have not been offset against derivative assets. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226016-175313 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109980867&loc=d3e41678-113959 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (d)(2) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312 + Details
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