Equity Investments - Additional Information (details) (Western Gas Partners [Member], USD $) In Millions, unless otherwise specified	Dec. 31, 2014
Fort Union [Member]
Schedule of Equity Method Investments [Line Items]
Equity investment difference between carrying and underlying value	$ 2.3
Equity investment ownership	14.81%
Rendezvous [Member]
Schedule of Equity Method Investments [Line Items]
Equity investment difference between carrying and underlying value	42.1
Equity investment ownership	22.00%
White Cliffs [Member]
Schedule of Equity Method Investments [Line Items]
Equity investment difference between carrying and underlying value	$ (9.0)
Equity investment ownership	10.00%
White Cliffs [Member] | Affiliated Entity [Member]
Schedule of Equity Method Investments [Line Items]
Equity investment ownership	0.40%