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Accumulated Other Comprehensive Loss - Reclassification Out of Accumulated Other Comprehensive Loss (Detail) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]                            
Amortization of actuarial loss $ (202,734)   $ (173,880)             $ (594,605) $ (500,329) $ (691,124) $ (556,009) $ (742,041)
Settlement loss (69,022)   (54,828)             (194,568) (162,359) (219,517) (205,409) (245,723)
Income (loss) before income taxes 33,218   26,926             96,608 74,277 89,284 (18,834) 77,265
Tax benefit (7,723)   (8,241)             (24,324) (22,973) 22,284 8,043 (24,131)
Net income 25,263 $ 60,194 18,421 $ 18,754 $ 13,269 $ 3,769 $ (2,503) $ (10,872) $ (3,036) 71,989 50,444 110,638 (12,642) 51,698
Pension and other post-retirement benefit plans:                            
Net of tax (439) [1]   (67) [1]             (704) [1] (199) [1] (2,856) (112) 269
Reclassification out of Accumulated Other Comprehensive Income                            
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]                            
Amortization of actuarial loss 116   100             243 299      
Settlement loss 484                 484        
Income (loss) before income taxes 600   100             727 299      
Tax benefit (161)   (35)             (192) (102)      
Net income $ 439   $ 65             $ 535 $ 197      
Pension and other post-retirement benefit plans:                            
Amortization of actuarial losses and net transition obligation                       397 362 340
Amortization of prior service costs                       2 2 2
Total before tax                       399 364 342
Tax benefit                       (141) (129) (120)
Net of tax                       $ 258 $ 235 $ 222
[1] Net of income tax (expense) benefit of $161 and $35 for the three months ended September 30, 2018 and 2017, respectively, and $123 and $102 for the nine months ended September 30, 2018 and 2017, respectively.