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Changes in Accumulated Other Comprehensive Income (Loss) (Parenthetical) (Detail) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Apr. 01, 2018
Dec. 31, 2018
Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Amounts reclassified from accumulated other comprehensive income (loss), tax $ (31) $ 60    
Other comprehensive income (loss), tax 11 18    
Reclassification of income tax effects from the Tax Reform Act, net of tax   769    
Reclassification of unrealized gains on equity securities, net of tax (20) (78)    
Foreign currency translation adjustments, tax 0 0 $ 0 $ 0
Unrealized (losses) gains on marketable securities, tax 36 418 521 1,815
Retirement plans prior service benefit, tax 1,092 1,028 $ 1,081 $ 932
Foreign Currency Translation Reclassification Adjustments        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Other comprehensive income (loss) before reclassifications, tax 0 0    
Amounts reclassified from accumulated other comprehensive income (loss), tax 0 0    
Other comprehensive income (loss), tax 0 0    
Reclassification of income tax effects from the Tax Reform Act, net of tax   0    
Reclassification of unrealized gains on equity securities, net of tax   0    
Unrealized (Losses) Gains on Marketable Securities        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Other comprehensive income (loss) before reclassifications, tax 577 718    
Amounts reclassified from accumulated other comprehensive income (loss), tax 20 78    
Other comprehensive income (loss), tax 557 640    
Reclassification of income tax effects from the Tax Reform Act, net of tax   691    
Reclassification of unrealized gains on equity securities, net of tax   (902)    
Unrealized (losses) gains on marketable securities, tax 20 78    
Amortization of Prior Service Credit        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Other comprehensive income (loss) before reclassifications, tax 0 0    
Amounts reclassified from accumulated other comprehensive income (loss), tax 11 18    
Other comprehensive income (loss), tax $ 11 18    
Reclassification of income tax effects from the Tax Reform Act, net of tax   78    
Reclassification of unrealized gains on equity securities, net of tax   $ 0