Derivative Instruments and Fair Value Measurements - Nonrecurring fair value measurements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Non-cash impairment loss	$ 0	$ 3,505	$ 0
Fair Value, Measurements, Nonrecurring
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 | AREC
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 | VestaCare, Inc.
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 | Bencap LLC
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 | Oil and gas properties
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments			62,784
Non-cash impairment loss			313
Fair Value, Measurements, Nonrecurring | Quoted Prices in Active Markets for Identical Assets and Liabilities (Level 1) | AREC
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		0
Fair Value, Measurements, Nonrecurring | Quoted Prices in Active Markets for Identical Assets and Liabilities (Level 1) | VestaCare, Inc.
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		0
Fair Value, Measurements, Nonrecurring | Quoted Prices in Active Markets for Identical Assets and Liabilities (Level 1) | Bencap LLC
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments			0
Fair Value, Measurements, Nonrecurring | Quoted Prices in Active Markets for Identical Assets and Liabilities (Level 1) | Oil and gas properties
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments			0
Fair Value, Measurements, Nonrecurring | Significant Unobservable Inputs (Level 3) | AREC
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		425
Fair Value, Measurements, Nonrecurring | Significant Unobservable Inputs (Level 3) | VestaCare, Inc.
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		0
Fair Value, Measurements, Nonrecurring | Significant Unobservable Inputs (Level 3) | Bencap LLC
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments			0
Fair Value, Measurements, Nonrecurring | Significant Unobservable Inputs (Level 3) | Oil and gas properties
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments			0
Fair Value, Measurements, Nonrecurring | Significant Unobservable Inputs (Level 3) | AREC
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		0
Fair Value, Measurements, Nonrecurring | Significant Unobservable Inputs (Level 3) | VestaCare, Inc.
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		$ 0
Fair Value, Measurements, Nonrecurring | Significant Unobservable Inputs (Level 3) | Bencap LLC
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments			0
Fair Value, Measurements, Nonrecurring | Significant Unobservable Inputs (Level 3) | Oil and gas properties
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments			$ 62,784