Annual report pursuant to Section 13 and 15(d)

ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Details)

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ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Beginning Balance $ 711,908 $ 653,924 $ 568,545
Change during 2019:      
Other comprehensive earnings/(loss) 49,969 (25,122) 24,119
Ending Balance 815,750 711,908 653,924
Interest Rate Swap [Member]      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Beginning Balance
Change during 2019:      
Before-tax amount (277)
Tax expense
Other comprehensive earnings/(loss) (277)
Ending Balance (277)
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Beginning Balance (46,091) (35,041) (49,200)
Change during 2019:      
Before-tax amount 75,449 (14,812) 18,980
Tax expense (29,553) 3,762 (4,821)
Other comprehensive earnings/(loss) 45,896 (11,050) 14,159
Ending Balance (195) (46,091) (35,041)
Accumulated Foreign Currency Adjustment Attributable to Parent [Member]      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Beginning Balance (24,987) (10,915) (20,875)
Change during 2019:      
Before-tax amount 4,350 (14,072) 9,960
Tax expense  
Other comprehensive earnings/(loss) 4,350 (14,072) 9,960
Ending Balance (20,637) (24,987) (10,915)
AOCI Attributable to Parent [Member]      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Beginning Balance (71,078) (45,956) (70,075)
Change during 2019:      
Before-tax amount 79,552 (28,884) 28,940
Tax expense (29,553) 3,762 (4,821)
Other comprehensive earnings/(loss) 49,969 (25,122) 24,119
Ending Balance $ (21,109) $ (71,078) $ (45,956)