Business Segments - Revenue Components of Mineral Resources Segment (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Revenue from External Customer [Line Items]
Segment revenues	$ 45,338		$ 47,089		$ 63,098
Mineral resources
Revenue from External Customer [Line Items]
Increase (Decrease) in Price Per Barrel, Percent	3.00%
Decrease in Revenues	(3,770)
Increase (Decrease) in Revenues, Percent	(27.00%)
Segment revenues	10,242	[1]	14,012	[1]	12,206	[1]
Decrease in Royalties Revenues	3,401
Increase (Decrease) in Production, Percent	29.00%
Mineral resources | Oil and gas
Revenue from External Customer [Line Items]
Segment revenues	7,810		11,075		9,876
Mineral resources | Rock aggregate
Revenue from External Customer [Line Items]
Segment revenues	964		920		886
Mineral resources | Cement
Revenue from External Customer [Line Items]
Segment revenues	819		758		621
Mineral resources | Land lease for oil exploration
Revenue from External Customer [Line Items]
Segment revenues	648		1,257		811
Mineral resources | Other
Revenue from External Customer [Line Items]
Segment revenues	$ 1		$ 2		$ 12

[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.