Unsecured Debt - Unsecured Private Placement Bonds (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Senior notes	$ 275,000	$ 315,000
Unsecured Bonds 4.50% [Member]
Debt Instrument [Line Items]
Maturity date	Sep. 30, 2017
Senior notes	$ 0	40,000
Coupon Rate	4.50%
Unsecured Bonds 4.92 % [Member]
Debt Instrument [Line Items]
Maturity date	Dec. 31, 2019
Senior notes	$ 75,000	75,000
Coupon Rate	4.92%
Unsecured Bonds 4.27% [Member]
Debt Instrument [Line Items]
Maturity date	Apr. 30, 2021
Senior notes	$ 100,000	100,000
Coupon Rate	4.27%
Unsecured Bonds 4.30% [Member]
Debt Instrument [Line Items]
Maturity date	Jun. 30, 2021
Senior notes	$ 50,000	50,000
Coupon Rate	4.30%
Unsecured Bonds 4.37% [Member]
Debt Instrument [Line Items]
Maturity date	Aug. 31, 2021
Senior notes	$ 50,000	$ 50,000
Coupon Rate	4.37%

Contractual interest rate for funds borrowed, under the debt agreement.

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 210.5-02.22(a)(1))
-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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- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ 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 210.5-02.22(a)(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Including the current and noncurrent portions, carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer). Senior note holders are paid off in full before any payments are made to junior note holders.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.16(a))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.16)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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