Unsecured Debt and Lines of Credit (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Senior Notes	$ 465,000us-gaap_SeniorNotes	$ 465,000us-gaap_SeniorNotes
Unsecured Bonds 4.36% [Member]
Debt Instrument [Line Items]
Maturity date	Mar. 31, 2016
Senior Notes	150,000	150,000
Debt Instrument, Interest Rate, Stated Percentage	4.36%	4.36%
Unsecured Bonds 4.5% [Member]
Debt Instrument [Line Items]
Maturity date	Sep. 30, 2017
Senior Notes	40,000	40,000
Debt Instrument, Interest Rate, Stated Percentage	4.50%	4.50%
Debt Instrument, Interest Rate, Effective Percentage	4.50%	4.50%
Unsecured Bonds 4.92 % [Member]
Debt Instrument [Line Items]
Maturity date	Dec. 31, 2019
Senior Notes	75,000	75,000
Debt Instrument, Interest Rate, Stated Percentage	4.92%	4.92%
Unsecured Bonds 4.27% [Member]
Debt Instrument [Line Items]
Maturity date	Apr. 30, 2021
Senior Notes	100,000	100,000
Debt Instrument, Interest Rate, Stated Percentage	4.27%	4.27%
Unsecured Bonds 4.30% [Member]
Debt Instrument [Line Items]
Maturity date	Jun. 30, 2021
Senior Notes	50,000	50,000
Debt Instrument, Interest Rate, Stated Percentage	4.30%	4.30%
Unsecured Bonds 4.37% [Member]
Debt Instrument [Line Items]
Maturity date	Aug. 31, 2021
Senior Notes	$ 50,000	$ 50,000
Debt Instrument, Interest Rate, Stated Percentage	4.37%	4.37%

The average effective interest rate during the reporting period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

Reference 3: 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))

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

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-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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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 SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

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-Section S99

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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 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.16)

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

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-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 16

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