Segment Information - Assets Allocated by Segment (Detail) - USD ($) $ in Thousands		Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Total Assets		$ 1,041,216	$ 1,258,199
Cash and cash equivalents		92,640	170,127	$ 170,606	$ 192,307
Deferred tax asset, net		1,118	40,624
Operating Segments [Member] | Real estate [Member]
Segment Reporting Information [Line Items]
Total Assets		713,912	654,774
Operating Segments [Member] | Oil And Gas [Member]
Segment Reporting Information [Line Items]
Total Assets		191,328	342,703
Operating Segments [Member] | Other natural resources [Member]
Segment Reporting Information [Line Items]
Total Assets		20,034	22,531
Items not allocated to segments [Member] | Assets not allocated to segments [Member]
Segment Reporting Information [Line Items]
Total Assets	[1]	$ 115,942	$ 238,191

[1] Assets not allocated to segments at third quarter-end 2015 principally consist of cash and cash equivalents of $92,640,000 and a net deferred tax asset of $1,118,000. Assets not allocated to segments at year-end 2014 principally consist of cash and cash equivalents of $170,127,000 and a net deferred tax asset of $40,624,000.