Annual report pursuant to Section 13 and 15(d)

Estimated Useful Lives of Assets (Parenthetical) (Detail)

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Estimated Useful Lives of Assets (Parenthetical) (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Property, Plant and Equipment [Line Items]    
Accumulated depreciation and amortization $ 1,500,535 $ 1,355,218
Property Under Capital Lease    
Property, Plant and Equipment [Line Items]    
Accumulated depreciation and amortization $ 200,683 $ 175,166