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Pension and other post-retirement benefits - Projected Benefit Obligations, Fair Value of Plan Assets, and Funded Status (Detail) - USD ($)
$ in Thousands
 6 Months Ended 12 Months Ended
Jun. 30, 2017
Dec. 31, 2018
Dec. 31, 2017
Change in projected benefit obligation      
Actuarial (gain) loss $ (2,354)    
Gain on curtailment 853    
Pension Plans      
Change in projected benefit obligation      
Projected benefit obligation, beginning of year 247,246 $ 523,743 $ 247,246
Projected benefit obligation assumed from business combination   0 256,486
Service cost   15,481 14,747
Interest cost   18,717 20,191
Actuarial (gain) loss   (29,845) 35,696
Contributions from retirees   0 0
Gain on curtailment   (1,875) (849)
Benefits paid   (49,429) (49,774)
Projected benefit obligation, end of year   476,792 523,743
Change in plan assets      
Fair value of plan assets, beginning of year 176,040 403,945 176,040
Plan assets acquired in business combination   0 184,510
Actual return on plan assets   (36,987) 63,250
Employer contributions   21,570 29,919
Benefits paid   (49,429) (49,774)
Fair value of plan assets, end of year   339,099 403,945
Unfunded status   (137,693) (119,798)
Non-current assets   0 0
Current liabilities   (872) (861)
Non-current liabilities   (136,821) (118,937)
Net amount recognized   (137,693) (119,798)
Other Postretirement Benefit Plans, Defined Benefit      
Change in projected benefit obligation      
Projected benefit obligation, beginning of year 61,888 176,975 61,888
Projected benefit obligation assumed from business combination   0 97,761
Service cost   5,791 4,838
Interest cost   6,727 6,642
Actuarial (gain) loss   (14,800) 10,263
Contributions from retirees   1,920 1,821
Gain on curtailment   0 (4)
Benefits paid   (8,288) (6,234)
Projected benefit obligation, end of year   168,325 176,975
Change in plan assets      
Fair value of plan assets, beginning of year $ 21,701 130,487 21,701
Plan assets acquired in business combination   0 91,532
Actual return on plan assets   (10,603) 19,733
Employer contributions   2,068 2,068
Benefits paid   (6,410) (4,547)
Fair value of plan assets, end of year   115,542 130,487
Unfunded status   (52,783) (46,488)
Non-current assets   3,161 3,936
Current liabilities   (850) (1,172)
Non-current liabilities   (55,094) (49,252)
Net amount recognized   $ (52,783) $ (46,488)