Employee Benefit Plans Employee Benefit Plans 8 (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Post-Retirement Medical Benefits
Employee benefit plans
Fair value of post-retirement plan assets	$ 10,538	$ 10,614	$ 10,723
Post-Retirement Medical Benefits | Level 1
Employee benefit plans
Fair value of post-retirement plan assets	10,538	10,614
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	360	31
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	4,072	4,182
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	6,106	6,401
Post-Retirement Medical Benefits | U.S. large cap funds | Level 2
Employee benefit plans
Fair value of post-retirement plan assets	0	0
Post-Retirement Medical Benefits | U.S. large cap funds | Level 3
Employee benefit plans
Fair value of post-retirement plan assets	0	0
Fair Value | Post-Retirement Medical Benefits
Employee benefit plans
Fair value of post-retirement plan assets	10,538	10,614
Fair Value | Post-Retirement Medical Benefits | Cash equivalents
Employee benefit plans
Fair value of post-retirement plan assets	360	31
Fair Value | Post-Retirement Medical Benefits | Fixed income fund
Employee benefit plans
Fair value of post-retirement plan assets	4,072	4,182
Fair Value | Post-Retirement Medical Benefits | U.S. large cap funds
Employee benefit plans
Fair value of post-retirement plan assets	$ 6,106	$ 6,401
Minimum | Fixed income fund
Employee benefit plans
Maturity period of investments	1 year
Maximum | Fixed income fund
Employee benefit plans
Maturity period of investments	20 years