Income taxes - Deferred Tax Liabilities and Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Liabilities
Accelerated depreciation	$ 80,101	$ 55,783
Accelerated amortization of deductible intangibles	667,015	663,545
Partnership investments including impairments	309,515	282,784
Other	7,570	9,057
Total deferred tax liabilities	1,064,201	1,011,169
Assets
Accrued compensation costs	32,361	28,119
Postretirement medical and life	78,318	73,470
Loss carryforwards	197,812	184,117
Other	36,465	26,735
Total deferred tax assets	344,956	312,441
Valuation allowance	209,939	184,413
Total net deferred tax (liabilities)	$ (929,184)	$ (883,141)