Derivative Instruments- Currency Swaps and Forwards (Details) € in Thousands, $ in Thousands	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2016 EUR (€)
Derivative [Line Items]
Derivative assets	$ 728	$ 20,571
Derivative liability, at fair value	(4,346)	(182,158)
Derivative instruments not accounted as hedges under GAAP
Derivative [Line Items]
Derivative assets	728	20,571
Derivative liability, at fair value	(4,346)	(182,158)
Total derivative instruments	(3,618)	(161,587)
Equity Option | Derivative instruments not accounted as hedges under GAAP
Derivative [Line Items]
Notional amount, assets	320,000	0
Notional amount, liabilities	(320,000)	0
Derivative assets	100	0
Derivative liability, at fair value	(50)	0
Foreign currency forwards | Derivative instruments not accounted as hedges under GAAP
Derivative [Line Items]
Notional amount, assets	0	784
Notional amount, liabilities	0	(1,532)
Derivative assets	0	34
Derivative liability, at fair value	0	(69)
Euro Member Countries, Euro | Foreign currency forwards | Derivative instruments not accounted as hedges under GAAP
Derivative [Line Items]
Notional amount, assets | €			€ 710
Notional amount, liabilities | €			(1,383)
Derivative notional amount, net | €			2,093
Euro Member Countries, Euro | Buy USD/Sell EUR currency forward | Derivative instruments not accounted as hedges under GAAP
Derivative [Line Items]
Notional amount, assets | €			710
Euro Member Countries, Euro | Buy EUR/Sell USD currency forward | Derivative instruments not accounted as hedges under GAAP
Derivative [Line Items]
Notional amount, assets | €			673
Euro Member Countries, Euro | Buy EUR/Sell USD currency forward | Derivative instruments not accounted as hedges under GAAP
Derivative [Line Items]
Notional amount, liabilities | €			€ (710)
United States of America, Dollars | Foreign currency forwards | Derivative instruments not accounted as hedges under GAAP
Derivative [Line Items]
Notional amount, assets		784
Notional amount, liabilities		(1,532)
Derivative assets		34
Derivative liability, at fair value		(69)
Derivative notional amount, net		2,316
Total derivative instruments		(35)
United States of America, Dollars | Buy USD/Sell EUR currency forward | Derivative instruments not accounted as hedges under GAAP
Derivative [Line Items]
Notional amount, assets		784
Total derivative instruments		34
United States of America, Dollars | Buy EUR/Sell USD currency forward | Derivative instruments not accounted as hedges under GAAP
Derivative [Line Items]
Notional amount, assets		735
Total derivative instruments		(23)
United States of America, Dollars | Buy EUR/Sell USD currency forward | Derivative instruments not accounted as hedges under GAAP
Derivative [Line Items]
Notional amount, liabilities		(797)
Total derivative instruments		(46)
Long | Equity Option
Derivative [Line Items]
Notional amount, assets	320,000
Derivative assets	100
Long | Us Treasury Futures
Derivative [Line Items]
Derivative assets	628	71
Derivative liability, at fair value		2,500
Notional Amount	480,000	56,900
Short | Equity Option
Derivative [Line Items]
Notional amount, liabilities	(320,000)
Derivative liability, at fair value	$ (50)
Short | Us Treasury Futures
Derivative [Line Items]
Notional Amount		$ 176,300