Investment Securities (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Investment Securities
Fair value	$ 5,063	$ 4,997
Cost	$ 5,103	5,024
Ultra-Short-Term Bond Fund
Investment Securities
Target investment allocation (as a percent)	75.00%
Maturity period of mutual funds	1 year
Non-investment grade rated holdings (as a percent)	2.00%
Fair value	$ 3,811	3,760
Cost	$ 3,825	3,767
Short-Term Bond Index Fund
Investment Securities
Target investment allocation (as a percent)	25.00%
Maturity period of mutual funds	3 years
Non-investment grade rated holdings (as a percent)	0.00%
Fair value	$ 1,252	1,237
Cost	$ 1,278	$ 1,257
AAA rated \| Ultra-Short-Term Bond Fund
Investment Securities
Investment holdings grade rated (as a percent)	42.00%
AAA rated \| Short-Term Bond Index Fund
Investment Securities
Investment holdings grade rated (as a percent)	70.00%

Represents target allocation of investments in mutual funds.

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- Definition

Represents maturity period of mutual funds.

+ References

No definition available.

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- Definition

Represents the percentage of holdings that are non-investment grade rated.

+ References

No definition available.

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Represents the percentage of investment holdings that are grade rated.

+ References

No definition available.

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- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

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- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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