Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

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

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Derivative Financial Instruments (Details) - Not designated as a hedge - Total return swap contracts $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($) contract	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) contract
Derivative Financial Instruments
Number of contracts \| contract	6		5
Notional value	$ 212,700		$ 194,400
Cash collateral with the counterparties	9,300		5,200
Level 2
Derivative Financial Instruments
Fair value - assets			4,968
Fair value - liabilities	2,188
Level 2 \| Investment and other income (loss)
Derivative Financial Instruments
Net gains (losses) recognized in income	(20,622)	$ 1,364
Level 2 \| Prepaid expenses and other current assets
Derivative Financial Instruments
Fair value - assets			$ 4,968
Level 2 \| Other current liabilities
Derivative Financial Instruments
Fair value - liabilities	$ 2,188