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Derivative Financial Instruments - Additional Information (Details) £ in Millions	Jun. 30, 2019 USD ($) derivative	Dec. 31, 2018 USD ($) derivative $ / unit	Dec. 31, 2018 GBP (£) derivative $ / unit
Derivative [Line Items]
Number of derivative contracts with contingent feature in a net liability position | derivative	0
Forward contracts
Derivative [Line Items]
Derivative, number of contracts | derivative		2	2
Cash collateral held from counterparty | $	$ 8,800,000
Cash collateral held at counterparty | $	$ 37,300,000
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 Liability, Notional Amount		$ 450,000,000.0	£ 325.3
Designated as Hedging Instrument | Net Investment Hedging | Forward contract expiring 2021
Derivative [Line Items]
Derivative Liability, Notional Amount		$ 400,000,000.0	£ 285.8
Designated as Hedging Instrument | Net Investment Hedging | Collar contract-call option | Minimum
Derivative [Line Items]
Derivative, option strike price (in dollars per pound sterling)		1.318	1.318
Designated as Hedging Instrument | Net Investment Hedging | Collar contract-call option | Maximum
Derivative [Line Items]
Derivative, option strike price (in dollars per pound sterling)		1.448	1.448
Designated as Hedging Instrument | Net Investment Hedging | Collar contract-put option | Minimum
Derivative [Line Items]
Derivative, option strike price (in dollars per pound sterling)		1.288	1.288
Designated as Hedging Instrument | Net Investment Hedging | Collar contract-put option | Maximum
Derivative [Line Items]
Derivative, option strike price (in dollars per pound sterling)		1.535	1.535

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Derivative, Net Liability Position, Number of Instruments

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Amount of obligation to return cash collateral under master netting arrangements that have not been offset against derivative assets.

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Amount of right to receive cash collateral under master netting arrangements that have not been offset against derivative liabilities.

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- Definition

The strike price on the foreign currency option contract such as a put option or a call option.

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Nominal or face amount used to calculate payments on the derivative liability.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The number of derivative instruments of a particular group held by the entity.

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