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Pension and other post-retirement benefits - Additional Information (Detail) - USD ($) $ in Thousands | 6 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2017 | Dec. 31, 2018 | Dec. 31, 2017
Employee Benefits Disclosure [Line Items] |  |  |
Defined contribution pension plan cost |  | $ 8,446 | $ 7,232
Accumulated benefit obligation for pension plan |  | 439,458 | 490,108
Actuarial (gain) loss | $ (2,354) |  |
Gain on curtailment | $ 853 |  |
Defined benefit plan, amounts recognized in other comprehensive income (loss), net prior service cost (credit) |  | 1,875 |
Reclassification of non-service costs |  |  | 9,035
Pension Plans |  |  |
Employee Benefits Disclosure [Line Items] |  |  |
Actuarial (gain) loss |  | (29,845) | 35,696
Gain on curtailment |  | (1,875) | (849)
Expected employer contributions for next year |  | 20,137 |
Other Postretirement Benefit Plans, Defined Benefit |  |  |
Employee Benefits Disclosure [Line Items] |  |  |
Actuarial (gain) loss |  | (14,800) | 10,263
Gain on curtailment |  | 0 | $ (4)
Expected employer contributions for next year |  | $ 5,562 |

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- Definition Amount of actuarial present value of benefits attributed to employee service rendered, excluding assumptions about future compensation level. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of gain (loss) from change in actuarial assumptions which (increases) decreases benefit obligation of defined benefit plan. Assumptions include, but are not limited to, interest, mortality, employee turnover, salary, and temporary deviation from substantive plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(4) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount, before tax, after reclassification adjustment, of (increase) decrease in accumulated other comprehensive income from prior service cost (credit) of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=84228862&loc=d3e637-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669646-108580 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of (increase) decrease in benefit obligation of defined benefit plan from event reducing expected years of future service of present employees or eliminating accrual of benefits for some or all future services of present employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(10) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of cost for defined contribution plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942 + Details
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