Derivative Instruments and Fair Value Measurements - Nonrecurring fair value measurements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Non-cash impairment loss	$ 3,505	$ 0	$ 0
Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Non-cash impairment loss	6,005	2,513
Fair Value, Measurements, Nonrecurring [Member] | AREC [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	425
Non-cash impairment loss	3,505
Fair Value, Measurements, Nonrecurring [Member] | VestaCare, Inc. [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0
Non-cash impairment loss	2,500
Fair Value, Measurements, Nonrecurring [Member] | Bencap LLC [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		0
Non-cash impairment loss		2,200
Fair Value, Measurements, Nonrecurring [Member] | Oil and Gas Properties [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		62,784	77,117
Non-cash impairment loss		313	12,082
Fair Value, Measurements, Nonrecurring [Member] | Quoted Prices in Active Markets for Identical Assets and Liabilities (Level 1) [Member] | AREC [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0
Fair Value, Measurements, Nonrecurring [Member] | Quoted Prices in Active Markets for Identical Assets and Liabilities (Level 1) [Member] | VestaCare, Inc. [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0
Fair Value, Measurements, Nonrecurring [Member] | Quoted Prices in Active Markets for Identical Assets and Liabilities (Level 1) [Member] | Bencap LLC [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		0
Fair Value, Measurements, Nonrecurring [Member] | Quoted Prices in Active Markets for Identical Assets and Liabilities (Level 1) [Member] | Oil and Gas Properties [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		0	0
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | AREC [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	425
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | VestaCare, Inc. [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Bencap LLC [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		0
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Oil and Gas Properties [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		0	0
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | AREC [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | VestaCare, Inc. [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	$ 0
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Bencap LLC [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		0
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Oil and Gas Properties [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		$ 62,784	$ 77,117