Property and Equipment - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Capitalized computer software development costs	$ 3,200	$ 3,200
Capitalized computer software development costs, accumulated depreciation and amortization	3,000	2,800
Valuation adjustments, amounts reclassified from property and equipment to other assets		1,300
Assets under capital lease included in Property and Equipment	1,403,268	1,277,592
Assets under capital lease, accumulated amortization	303,482	245,162
Assets Held under Capital Leases [Member]
Property, Plant and Equipment [Line Items]
Assets under capital lease included in Property and Equipment	31,800	31,800
Assets under capital lease, accumulated amortization	15,400	$ 14,600
ASU 2015-16 [Member]
Property, Plant and Equipment [Line Items]
Valuation adjustments, amounts reclassified from property and equipment to other assets	$ 1,300