Business Segments - Revenue Components of Mineral Resources Segment (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Revenue from External Customer [Line Items]
Segment revenues	$ 47,089		$ 63,098		$ 35,513
Mineral resources [Member]
Revenue from External Customer [Line Items]
Segment revenues	14,012	[1]	12,206	[1]	6,362	[1]
Mineral resources [Member] | Oil and gas [Member]
Revenue from External Customer [Line Items]
Segment revenues	11,075		9,876		4,960
Mineral resources [Member] | Rock aggregate [Member]
Revenue from External Customer [Line Items]
Segment revenues	920		886		710
Mineral resources [Member] | Cement [Member]
Revenue from External Customer [Line Items]
Segment revenues	758		621		496
Mineral resources [Member] | Land lease for oil exploration [Member]
Revenue from External Customer [Line Items]
Segment revenues	1,257		811		196
Mineral resources [Member] | Other [Member]
Revenue from External Customer [Line Items]
Segment revenues	$ 2		$ 12		$ 0

[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.