Asset Retirement Obligations - Additional Information (Detail)	12 Months Ended
Dec. 31, 2018 USD ($) Structure	Dec. 31, 2017 USD ($)
Asset Retirement Obligation Disclosure [Abstract]
Estimated CARO ranges per structure, minimum	$ 3,000,000
Estimated CARO ranges per structure, maximum	$ 28,000,000
Number of structures required to demolish \| Structure	24
Current liability	$ 122,000,000	$ 108,000,000

Current portion of the carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

Name:	us-gaap_AssetRetirementObligationCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant