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(Parenthetical) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Statement of Comprehensive Income [Abstract]      
Unrealized gain (loss) on investments, tax $ 0 $ 0 $ 0
Noncash Titan Europe Plc gain, tax 0 0 0
Pension liaiblity adjustments, tax $ 151 $ 215 $ (439)