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Commitments and Contingencies (Narrative) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Contractual Obligation Additional Investment in Alternative or Other Investments $ 221,000    
Purchase amount of annuities 18,500    
Rental expense for operating leases for office space, equipment, and vehicles $ 10,800 $ 12,300 $ 11,700
Capital Lease Agreements Expiration Period 2019    
Other Investments [Member]      
Remaining Commitment $ 220,999 $ 143,686