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Fair Value Measurements (Fair Value of Available-for-sale Securities) (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]    
Cost $ 830 $ 830
Gross unrealized gains 0 0
Gross unrealized losses 605 636
Available-for-sale Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Estimated fair value 225 194
Available-for-sale Securities [Member] | Level 1 [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Estimated fair value $ 225 $ 194