Employee Benefit Plans Employee Benefit Plans 8 (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Benefits
Employee benefit plans
Defined Benefit Plan Number of Highest Consecutive Years Average Earnings Used in Computing Retirement Benefit	5 years
Fair value of post-retirement plan assets	$ 192,477	$ 162,529	$ 173,648
Pension Benefits | Cash equivalents
Employee benefit plans
Fair value of post-retirement plan assets	600	590
Pension Benefits | Fixed income fund
Employee benefit plans
Fair value of post-retirement plan assets	74,123	70,642
Pension Benefits | U.S. large cap funds
Employee benefit plans
Fair value of post-retirement plan assets	47,132	46,133
Post-Retirement Medical Benefits
Employee benefit plans
Fair value of post-retirement plan assets	11,271	10,010	$ 11,053
Post-Retirement Medical Benefits | Level 1
Employee benefit plans
Fair value of post-retirement plan assets	11,271	10,010
Post-Retirement Medical Benefits | Level 2
Employee benefit plans
Fair value of post-retirement plan assets	0	0
Post-Retirement Medical Benefits | Level 3
Employee benefit plans
Fair value of post-retirement plan assets	0	0
Post-Retirement Medical Benefits | Cash equivalents | Level 1
Employee benefit plans
Fair value of post-retirement plan assets	69	263
Post-Retirement Medical Benefits | Cash equivalents | Level 2
Employee benefit plans
Fair value of post-retirement plan assets	0	0
Post-Retirement Medical Benefits | Cash equivalents | Level 3
Employee benefit plans
Fair value of post-retirement plan assets	0	0
Post-Retirement Medical Benefits | Fixed income fund | Level 1
Employee benefit plans
Fair value of post-retirement plan assets	4,279	3,871
Post-Retirement Medical Benefits | Fixed income fund | Level 2
Employee benefit plans
Fair value of post-retirement plan assets	0	0
Post-Retirement Medical Benefits | Fixed income fund | Level 3
Employee benefit plans
Fair value of post-retirement plan assets	0	0
Post-Retirement Medical Benefits | U.S. large cap funds | Level 1
Employee benefit plans
Fair value of post-retirement plan assets	6,923	5,876
Post-Retirement Medical Benefits | U.S. large cap funds | Level 2
Employee benefit plans
Fair value of post-retirement plan assets	0
Post-Retirement Medical Benefits | U.S. large cap funds | Level 3
Employee benefit plans
Fair value of post-retirement plan assets	0
Fair Value | Post-Retirement Medical Benefits
Employee benefit plans
Fair value of post-retirement plan assets	11,271	10,010
Fair Value | Post-Retirement Medical Benefits | Cash equivalents
Employee benefit plans
Fair value of post-retirement plan assets	69	263
Fair Value | Post-Retirement Medical Benefits | Fixed income fund
Employee benefit plans
Fair value of post-retirement plan assets	4,279	3,871
Fair Value | Post-Retirement Medical Benefits | U.S. large cap funds
Employee benefit plans
Fair value of post-retirement plan assets	$ 6,923	$ 5,876
Minimum | Post-Retirement Medical Benefits | Fixed income fund
Employee benefit plans
Investments Maturity Period	1 year
Maximum | Post-Retirement Medical Benefits | Fixed income fund
Employee benefit plans
Investments Maturity Period	20 years