Business Segments - Revenue Components of Mineral Resources Segment (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Revenue from External Customer [Line Items]
Segment revenues	$ 51,252		$ 45,338		$ 47,089
Mineral resources
Revenue from External Customer [Line Items]
Decrease in Revenues	6,013
Decrease in revenues, percent	59.00%
Segment revenues	16,255	[1]	10,242	[1]	14,012	[1]
Decrease in royalties revenues	3,401
Decrease in production, percent	29.00%
Decrease in price per barrel, percent	3.00%
Mineral resources | Oil and gas
Revenue from External Customer [Line Items]
Segment revenues	6,096		7,810		11,075
Mineral resources | Rock aggregate
Revenue from External Customer [Line Items]
Segment revenues	1,216		964		920
Mineral resources | Cement
Revenue from External Customer [Line Items]
Segment revenues	1,043		819		758
Mineral resources | Land lease for oil exploration
Revenue from External Customer [Line Items]
Segment revenues	198		648		1,257
Mineral resources | Water sales
Revenue from External Customer [Line Items]
Segment revenues	7,702		0		0
Mineral resources | Other
Revenue from External Customer [Line Items]
Segment revenues	$ 0		$ 1		$ 2

[1] During the fourth quarter of 2012, the Company evaluated its operations and determined that an additional segment should be reported, Mineral resources. Mineral resources collects royalty income from oil and gas leases, rock and aggregate leases, and from a cement company.