CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Comprehensive income                      
Net earnings (loss) $ 28,973 $ 30,436 $ 23,967 $ 28,291 $ 29,819 $ 7,376 $ 31,393 $ (4,508) $ 111,667 $ 103,856 $ 64,080
Unrealized foreign currency translation adjustment, net of tax                 (3,881) 1,372 978
Pension and other benefit adjustments:                      
Net loss arising during the period, net of tax                 (293) 166 (13,543)
Prior service credit arising during the period, net of tax                   746  
Amortization of net (gain) loss reclassified into general and administrative: other, net of tax                 16 (1,679) (844)
Amortization of prior service credit reclassified into general and administrative: other, net of tax                   (1,762) (1,016)
Curtailment gain reclassified into general and administrative: other, net of tax                   (7,239)  
Settlement gain reclassified into general and administrative: other, net of tax                 (3) (196)  
Marketable securities:                      
Unrealized net holding gain (loss) arising during the period, net of tax                 589 (1,056) 2,627
Realized net (gain) loss reclassified into investment income, net of tax                 (1,816) (156) (31)
Equity method investees' cash flow hedge:                      
Unrealized net holding gain arising during the period, net of tax                 (309) (693) (59)
Realized net loss reclassified into equity in earnings of non-consolidated entities, net of tax                 342 457 528
Other comprehensive income (loss), net of tax                 (5,355) (10,040) (11,360)
Total comprehensive income                 $ 106,312 $ 93,816 $ 52,720