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Consolidated Statements of Comprehensive Income Parenthetical - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Statement of Comprehensive Income [Abstract]      
Actuarial gain, tax $ 1,792 $ 3,703 $ 3,235
Amortization of actuarial loss, tax 651 957 1,880
Effects of settlements, tax $ 0 $ 1,598 $ 193