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Discontinued Operations - Additional Information (Detail) - USD ($)		3 Months Ended		6 Months Ended
	Jan. 08, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Discontinued operations		$ 0	$ 540,485,000	$ 0	$ 539,538,000
January 2017 Corporate Restructuring [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Restructuring activity, announcement date				Jan. 08, 2017
Reduction in headcount, percent	30.00%
January 2017 Corporate Restructuring [Member] \| Discontinued Operations [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Restructuring expenses				$ 9,500,000
January 2017 Corporate Restructuring [Member] \| Contractual Termination Benefits [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Restructuring expenses			$ 4,200,000		$ 9,500,000
January 2017 Corporate Restructuring [Member] \| One-time Employee Termination Benefits [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Restructuring activities, description				One-time employee termination benefits are comprised of severance, benefits and related costs, all of which are expected to result in cash expenditures.