Commitments and Contingencies - Non-Core Assets Restructuring Costs (Detail)	9 Months Ended
	Sep. 30, 2016 USD ($)
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Restructuring charges	$ 2,904,000
One-time Termination Benefits | Real estate
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Restructuring charges	1,422,000
One-time Termination Benefits | Other natural resources
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Restructuring charges	164,000
General and Administrative Expense | One-time Termination Benefits
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Restructuring charges	$ 486,000