Fair Value Measurements - Nonrecurring Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
Jun. 30, 2013	Jun. 30, 2012
Nonrecurring Fair Value Measurements [Line Items]
Liabilities incurred in current period	$ 262	$ 416
Significant unobservable inputs (Level 3) \| Nonrecurring fair value measurements
Nonrecurring Fair Value Measurements [Line Items]
Annual inflation rate	2.95%	2.95%
Credit-adjusted risk-free interest rate	7.10%	7.50%
Liabilities incurred in current period	$ 262	$ 416

Amount of asset retirement obligations incurred during the period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 143

-Paragraph 22

-Subparagraph c(1)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Name:	us-gaap_AssetRetirementObligationLiabilitiesIncurred
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration