Derivatives (Derivatives Not Designated as Hedging Instruments - Commodity Contracts) (Details) MMBTU in Thousands, Boe in Thousands, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($) MMBTU Boe	Dec. 31, 2018 USD ($) MMBTU Boe
Derivative [Line Items]
Derivative assets - Fair value	$ 135,782	$ 107,816
Derivative liabilities - Fair value	109,587	118,305
Derivatives not designated as hedging instruments
Derivative [Line Items]
Derivative assets - Fair value	135,782	107,816
Derivative liabilities - Fair value	104,909	111,828
Derivatives not designated as hedging instruments | Commodity contracts
Derivative [Line Items]
Derivative assets - Fair value	7,239	14,422
Derivative liabilities - Fair value	8,016	23,068
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Commodity contracts
Derivative [Line Items]
Derivative assets - Fair value	4,906	1,127
Derivative liabilities - Fair value	$ 4,091	$ 22,687
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Crude oil | Written options
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	307	524
Derivative assets - Fair value	$ 442	$ 0
Derivative liabilities - Fair value	$ 303	$ 2,628
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Crude oil | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	2,394	872
Derivative assets - Fair value	$ 2,800	$ 0
Derivative liabilities - Fair value	$ 282	$ 3,772
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Crude oil | Swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	1,477	1,111
Derivative assets - Fair value	$ 719	$ 0
Derivative liabilities - Fair value	$ 2,484	$ 14,278
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Crude oil | Commodity contracts
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	4,178	2,507
Derivative assets - Fair value	$ 3,961	$ 0
Derivative liabilities - Fair value	$ 3,069	$ 20,678
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Natural gas | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	6,241	3,063
Derivative assets - Fair value	$ 128	$ 78
Derivative liabilities - Fair value	$ 19	$ 152
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Natural gas | Swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	14,438	11,659
Derivative assets - Fair value	$ 817	$ 1,049
Derivative liabilities - Fair value	$ 1,003	$ 1,857
Commodity contracts | Derivatives not designated as hedging instruments | Customer Counterparty | Natural gas | Commodity contracts
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	20,679	14,722
Derivative assets - Fair value	$ 945	$ 1,127
Derivative liabilities - Fair value	1,022	2,009
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Commodity contracts
Derivative [Line Items]
Derivative assets - Fair value	2,333	13,295
Derivative liabilities - Fair value	$ 3,925	$ 381
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Crude oil | Purchased options
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	307	524
Derivative assets - Fair value	$ 163	$ 2,251
Derivative liabilities - Fair value	$ 424	$ 0
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Crude oil | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	2,394	872
Derivative assets - Fair value	$ 254	$ 3,225
Derivative liabilities - Fair value	$ 2,758	$ 0
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Crude oil | Swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	1,477	1,111
Derivative assets - Fair value	$ 832	$ 5,799
Derivative liabilities - Fair value	$ 321	$ 0
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Crude oil | Commodity contracts
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	4,178	2,507
Derivative assets - Fair value	$ 1,249	$ 11,275
Derivative liabilities - Fair value	$ 3,503	$ 0
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Natural gas | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	6,241	3,063
Derivative assets - Fair value	$ 15	$ 151
Derivative liabilities - Fair value	$ 117	$ 64
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Natural gas | Swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	14,438	11,659
Derivative assets - Fair value	$ 1,069	$ 1,869
Derivative liabilities - Fair value	$ 305	$ 317
Commodity contracts | Derivatives not designated as hedging instruments | Financial Counterparty | Natural gas | Commodity contracts
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	20,679	14,722
Derivative assets - Fair value	$ 1,084	$ 2,020
Derivative liabilities - Fair value	$ 422	$ 381