Segment Information - Segment Revenues and Earnings (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Total revenues	$ 22,599,000	$ 22,305,000
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	4,600,000	40,998,000
Gain on sale of assets	2,746,000	74,215,000
General and Administrative Expense	(3,745,000)	(4,691,000)
Interest Expense	(2,136,000)	(2,235,000)
Other corporate interest and other income	1,652,000	676,000
Asset impairments	0	37,900,000
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	22,599,000	22,305,000
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	9,150,000	47,902,000
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	(4,550,000)	(6,904,000)
General and Administrative Expense	(3,653,000)	(4,028,000)
Share-based and long-term incentive compensation expense	(136,000)	(895,000)
Interest Expense	(2,136,000)	(2,235,000)
Other corporate interest and other income	1,375,000	254,000
Real Estate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	22,575,000	20,752,000
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	9,703,000	10,473,000
Other [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	24,000	1,553,000
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	$ (553,000)	37,429,000
the Mineral Companies | Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Disposal group, consideration		85,700,000
Gain on sale of assets		74,215,000
Asset impairments		$ 37,900,000