Employee Benefit Plans - Pension Plan (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Employee Benefit Plans
Projected benefit obligation	$ 21,489	$ 20,086	$ 22,217	$ 21,489	$ 20,086
Fair value of plan assets				17,099	15,800
Net unfunded pension liability				4,390	4,286
Actual return on plan assets	3,900	(700)
Components of Change in Benefit Obligation
Benefit obligation at beginning of year	20,086	22,217	20,631
Interest cost	767	714	724
Actuarial (gain) loss	3,839	(1,347)	2,392
Benefits paid	(3,203)	(1,498)	(1,530)
Benefit obligation at end of year	21,489	20,086	$ 22,217
Components of estimated future benefit payments
2020				1,647
2021				1,072
2022				1,281
2023				920
2024				942
2025-2029				7,782
Total				13,644
Global Pension Plan
Employee Benefit Plans
Projected benefit obligation	17,030	16,213		17,030	16,213
Fair value of plan assets				14,267	13,372
Net unfunded pension liability				2,763	2,841
Components of Change in Benefit Obligation
Benefit obligation at beginning of year	16,213
Benefit obligation at end of year	$ 17,030	$ 16,213
Components of weighted-average actuarial assumptions
Discount rate	3.00%	4.10%	3.40%
Expected return on plan assets	7.00%	7.00%	7.00%
Components of estimated future benefit payments
Contributions made by the General Partner, the Partnership and its subsidiaries to the Pension Plans	$ 600	$ 400	$ 400
GMG Pension Plan
Employee Benefit Plans
Projected benefit obligation	4,459	3,873		4,459	3,873
Fair value of plan assets				2,832	2,428
Net unfunded pension liability				$ 1,627	$ 1,445
Components of Change in Benefit Obligation
Benefit obligation at beginning of year	3,873
Benefit obligation at end of year	$ 4,459	$ 3,873
Components of weighted-average actuarial assumptions
Discount rate	3.20%	4.20%	3.60%
Expected return on plan assets	7.00%	7.00%	7.00%