Employee Benefit Plans 9 (Details) - Post-Retirement Medical Benefits - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Employee benefit plans
Fair value of post-retirement plan assets	$ 12,313	$ 11,271	$ 10,010
Fair Value
Employee benefit plans
Fair value of post-retirement plan assets	12,312	11,271
Level 1
Employee benefit plans
Fair value of post-retirement plan assets	12,312	11,271
Level 2
Employee benefit plans
Fair value of post-retirement plan assets	0	0
Level 3
Employee benefit plans
Fair value of post-retirement plan assets	0	0
Cash equivalents | Fair Value
Employee benefit plans
Fair value of post-retirement plan assets	169	69
Cash equivalents | Level 1
Employee benefit plans
Fair value of post-retirement plan assets	169	69
Cash equivalents | Level 2
Employee benefit plans
Fair value of post-retirement plan assets	0	0
Cash equivalents | Level 3
Employee benefit plans
Fair value of post-retirement plan assets	0	0
Fixed income fund | Fair Value
Employee benefit plans
Fair value of post-retirement plan assets	4,436	4,279
Fixed income fund | Level 1
Employee benefit plans
Fair value of post-retirement plan assets	4,436	4,279
Fixed income fund | Level 2
Employee benefit plans
Fair value of post-retirement plan assets	0	0
Fixed income fund | Level 3
Employee benefit plans
Fair value of post-retirement plan assets	0	0
U.S. large cap funds | Fair Value
Employee benefit plans
Fair value of post-retirement plan assets	7,707	6,923
U.S. large cap funds | Level 1
Employee benefit plans
Fair value of post-retirement plan assets	7,707	$ 6,923
U.S. large cap funds | Level 2
Employee benefit plans
Fair value of post-retirement plan assets	0
U.S. large cap funds | Level 3
Employee benefit plans
Fair value of post-retirement plan assets	$ 0