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Employee Benefit Plans Employee Benefit Plans 8 (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Post-Retirement Medical Benefits
Employee benefit plans
Fair value of post-retirement plan assets	$ 11,053	$ 10,538	$ 10,614
Post-Retirement Medical Benefits | Level 1
Employee benefit plans
Fair value of post-retirement plan assets	11,060	10,538
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	189	360
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	4,364	4,072
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	6,507	6,106
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	11,060	10,538
Fair Value | Post-Retirement Medical Benefits | Cash equivalents
Employee benefit plans
Fair value of post-retirement plan assets	189	360
Fair Value | Post-Retirement Medical Benefits | Fixed income fund
Employee benefit plans
Fair value of post-retirement plan assets	4,364	4,072
Fair Value | Post-Retirement Medical Benefits | U.S. large cap funds
Employee benefit plans
Fair value of post-retirement plan assets	$ 6,507	$ 6,106
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

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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