Debt - Debt Obligations (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Jun. 30, 2013
Long-term debt
Principal Balances, Total debt	$ 89,094	$ 79,928
Principal Balances, amounts due within one year	(56,097)	(4,811)
Principal Balances, long-term debt	32,997	75,117
Unamortized Debt Financing Costs, Total debt	209	213
Unamortized Debt Financing Costs, amounts due within one year	(84)	(99)
Unamortized Debt Financing Costs, long-term	125	114
Balance, Net of Financing Costs, Total debt	88,885	79,715
Balance, Net of Financing Costs, amounts due within one year	(56,013)	(4,712)
Balance, Net of Financing Costs, long-term debt	32,872	75,003
Summary of aggregate maturities of debt obligations
2020	56,097
2021	6,340
2022	6,515
2023	11,500
2024	8,642
Principal Balances, Total debt	89,094	79,928
7.5% Notes
Long-term debt
Principal Balances, Total debt	50,000	50,000
Unamortized Debt Financing Costs, Total debt	17	58
Balance, Net of Financing Costs, Total debt	$ 49,983	$ 49,942
Interest rate (as a percent)	7.50%	7.50%	7.50%
Summary of aggregate maturities of debt obligations
2020	$ 50,000
Principal Balances, Total debt	50,000	$ 50,000
5.25% Notes
Long-term debt
Principal Balances, Total debt		28,904
Unamortized Debt Financing Costs, Total debt		155
Balance, Net of Financing Costs, Total debt		28,749
Summary of aggregate maturities of debt obligations
Principal Balances, Total debt		28,904
NG Advantage debt
Long-term debt
Principal Balances, Total debt	33,898
Unamortized Debt Financing Costs, Total debt	192
Balance, Net of Financing Costs, Total debt	33,706
Summary of aggregate maturities of debt obligations
2020	5,857
2021	6,092
2022	6,300
2023	11,407
2024	4,242
Principal Balances, Total debt	33,898
SG Facility
Long-term debt
Principal Balances, Total debt	4,400
Balance, Net of Financing Costs, Total debt	4,400
Summary of aggregate maturities of debt obligations
2024	4,400
Principal Balances, Total debt	4,400
Other debt
Long-term debt
Principal Balances, Total debt	796	1,024
Balance, Net of Financing Costs, Total debt	796	1,024
Summary of aggregate maturities of debt obligations
2020	240
2021	248
2022	215
2023	93
Principal Balances, Total debt	$ 796	$ 1,024