Income taxes - Deferred Tax Liabilities and Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Liabilities
Accelerated depreciation	$ 40,568	$ 79,986
Accelerated amortization of deductible intangibles	420,301	658,651
Partnership investments including impairments	0	40,907
Other	5,255	3,924
Total deferred tax liabilities	466,124	783,468
Assets
Accrued compensation costs	15,133	31,929
Pension and postretirement medical and life	39,769	78,318
Loss carryforwards	132,214	197,812
Other	33,116	36,428
Total deferred tax assets	220,232	344,487
Valuation allowance	136,418	209,939
Total net deferred tax (liabilities)	$ (382,310)	$ (648,920)