Variable Interest Entities - Royalty Trusts - Open Oil and Natural Gas Commodity Derivative Contracts (Details)	12 Months Ended
Dec. 31, 2013 MBbls
Oil price swaps \| January 2014 - December 2014
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional (Oil in MBbl/Natural Gas in MMcf)	8,813
Weighted Avg. Fixed Price (Oil in USD/bbl, Natural Gas in USD/mcf)	92.98
Oil price swaps \| January 2015 - December 2015
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional (Oil in MBbl/Natural Gas in MMcf)	7,979
Weighted Avg. Fixed Price (Oil in USD/bbl, Natural Gas in USD/mcf)	86.13
Natural gas collars \| January 2014 - December 2014
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional (Oil in MBbl/Natural Gas in MMcf)	937
Collar range, minimum	4.00
Collar range, maximum	7.78
Natural gas collars \| January 2015 - December 2015
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional (Oil in MBbl/Natural Gas in MMcf)	1,010
Collar range, minimum	4.00
Collar range, maximum	8.55
Royalty Trusts \| Oil price swaps \| January 2014 - December 2014
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional (Oil in MBbl/Natural Gas in MMcf)	1,862
Weighted Avg. Fixed Price (Oil in USD/bbl, Natural Gas in USD/mcf)	100.70
Royalty Trusts \| Oil price swaps \| January 2015 - December 2015
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional (Oil in MBbl/Natural Gas in MMcf)	630
Weighted Avg. Fixed Price (Oil in USD/bbl, Natural Gas in USD/mcf)	101.03
Royalty Trusts \| Natural gas collars \| January 2014 - December 2014
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional (Oil in MBbl/Natural Gas in MMcf)	937
Collar range, minimum	4.00
Collar range, maximum	7.78
Royalty Trusts \| Natural gas collars \| January 2015 - December 2015
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional (Oil in MBbl/Natural Gas in MMcf)	1,010
Collar range, minimum	4.00
Collar range, maximum	8.55
Royalty Trusts \| Novated Contract \| Oil price swaps \| January 2014 - December 2014
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional (Oil in MBbl/Natural Gas in MMcf)	991
Weighted Avg. Fixed Price (Oil in USD/bbl, Natural Gas in USD/mcf)	100.79
Royalty Trusts \| Novated Contract \| Oil price swaps \| January 2015 - March 2015
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional (Oil in MBbl/Natural Gas in MMcf)	141
Weighted Avg. Fixed Price (Oil in USD/bbl, Natural Gas in USD/mcf)	100.90

Variable Interest Entities - Royalty Trusts - Open Oil and Natural Gas Commodity Derivative Contracts (Details)

12 Months Ended

Dec. 31, 2013

MBbls

Oil price swaps | January 2014 - December 2014

Derivative Instruments and Hedging Activities Disclosures [Line Items]

Notional (Oil in MBbl/Natural Gas in MMcf)

8,813

Weighted Avg. Fixed Price (Oil in USD/bbl, Natural Gas in USD/mcf)

92.98

Oil price swaps | January 2015 - December 2015

Derivative Instruments and Hedging Activities Disclosures [Line Items]