Derivative Financial Instruments (Details) - Not designated as a hedge - Total return swap contracts $ in Thousands	12 Months Ended
Dec. 31, 2019 USD ($) contract	Dec. 31, 2018 USD ($) contract
Derivative Financial Instruments
Number of contracts \| contract	14	5
Notional value	$ 228,200	$ 194,400
Cash collateral with the counterparties	3,700	5,200
Level 2
Derivative Financial Instruments
Net total return swap (liability) asset	(3,990)	4,968
Investment and other income (loss) \| Level 2
Derivative Financial Instruments
Net (losses) gains recognized in income	(38,240)	15,163
Prepaid expenses and other current assets \| Level 2
Derivative Financial Instruments
Fair value - assets		$ 4,968
Other current liabilities \| Level 2
Derivative Financial Instruments
Fair value - liabilities	$ 3,990

Fair values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments.

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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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Fair value as of the balance sheet date of derivative instrument not designated as hedging instrument and classified as an asset.

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Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments.

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Nominal or face amount used to calculate payment on derivative.

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