Income taxes - Deferred Tax Liabilities and Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Deferred tax liabilities
Accelerated depreciation	$ 67,479	$ 62,951
Accelerated amortization of deductible intangibles	536,740	524,697
Right-of-use assets for operating leases	24,220	25,615
Other	3,322	3,677
Total deferred tax liabilities	631,761	616,940
Deferred tax assets
Accrued compensation costs	18,559	16,180
Pension and post-retirement medical and life	25,523	35,192
Loss carryforwards	38,348	38,686
Operating lease liabilities	25,319	26,008
Other	37,239	30,914
Total deferred tax assets	144,988	146,980
Deferred tax asset valuation allowance	43,467	45,661	$ 125,894	$ 136,418
Total net deferred tax (liabilities)	$ (530,240)	$ (515,621)