Income Taxes - Deferred Income Taxes Related to Temporary Differences between Tax Bases of Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax liabilities:
Accelerated depreciation	$ 195,694	$ 293,141
Prepaid costs currently deductible	4,152	6,572
Other	4,405	5,747
Total deferred tax liabilities	204,251	305,460
Deferred tax assets:
Accrued costs not yet deductible	7,880	9,785
Allowance for doubtful accounts	484	809
Deferred revenues	213	965
Share-based compensation	1,045	1,882
Total deferred tax assets, net of valuation allowance of $0.8 million in 2017 and $0.7 million in 2016	9,622	13,441
Deferred income taxes, net	$ 194,629	$ 292,019