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Fair Value Measurements and Derivative Instruments (Unrealized Holding Losses) (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Fair Value    
Less than 12 continuous months $ 8,518 $ 48,318
Greater than 12 continuous months 133,732 177,331
Total 142,250 225,649
Gross unrealized losses    
Less than 12 continuous months 46 847
Greater than 12 continuous months 2,200 4,355
Total $ 2,246 $ 5,202