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Fair Value Measurements and Derivative Instruments (Unrealized Holding Losses) (Details) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Fair Value Disclosures [Abstract]    
Less than 12 continuous months $ 502 $ 48,318
Greater than 12 continuous months 56,970 177,331
Total 57,472 225,649
Gross unrealized losses    
Less than 12 continuous months 2 847
Greater than 12 continuous months 670 4,355
Total $ 672 $ 5,202