Debt (Details Textual) (USD $)	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011
Debt (Textual) [Abstract]
Offer and sale of various securities	$ 300,000
Previous Unsecured Bank Credit Facility [Member]
Debt (Textual) [Abstract]
Repayment of balance outstanding	38,000,000
Expiry date for agreement	Dec. 19, 2011
Current Unsecured Bank Credit Facility [Member]
Debt (Textual) [Abstract]
Unsecured borrowings	150,000,000
Expiry date for agreement	Oct. 06, 2015
Interest rate variable spread percentage	1.25%
Effective interest rate	1.56%
Current borrowing under the Current Bank Credit Facility	38,000,000
Variable commitment fee unused portion	0.15%
Senior Notes 2015 [Member]
Debt (Textual) [Abstract]
Unaccrued discount	65,000	92,000
Senior Notes	6.05%
Aggregate principal amount	75,000
Discount on issue	0.357%
Effective yield	6.098%
Percentage of principal amount of senior notes for redemption	100.00%
Treasury yield basis point	0.30%
Senior Notes 2015 [Member] \| Senior Note 6.05 % [Member]
Debt (Textual) [Abstract]
Maturity date of Senior note	Jun. 15, 2015
Senior Notes 2016 [Member]
Debt (Textual) [Abstract]
Unaccrued discount	126,000	164,000
Senior Notes	6.85%
Aggregate principal amount	$ 125,000
Maturity date of Senior note	Apr. 15, 2016
Discount on issue	0.305%
Effective yield	6.893%
Percentage of principal amount of senior notes for redemption	100.00%
Treasury yield basis point	0.30%

Debt instrument percentage of discount on issuance.

No definition available.

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- Details

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- Definition

Maximum amount covered under universal shelf registration for additional capital borrowings.

+ References

No definition available.

+ Details

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- Definition

Percentage of principal amount of senior notes for redemption.

+ References

No definition available.

+ Details

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- Definition

Percentage spread on treasury rate used to calculate redemption price.

+ References

No definition available.

+ Details

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- Definition

Variable commitment fee percentage charged on unused portion of bank credit facility.

+ References

No definition available.

+ Details

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- Definition

The percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

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- Definition

The stated principal amount of the debt instrument at time of issuance, which may vary from the carrying amount because of unamortized premium or discount.

+ References

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

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 55

-Paragraph 8

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-Publisher AICPA

-Name Accounting Principles Board Opinion (APB)

-Number 21

-Paragraph 16, 20

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

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- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 2

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

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-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

Interest rate stated in the contractual debt agreement.

+ References

+ Details

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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

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22(a)(2))

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-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 129

-Paragraph 2, 4

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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- Definition

The amount of debt discount that was originally recognized at the issuance of the instrument that has yet to be amortized.

+ References

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

-Publisher AICPA

-Name Accounting Principles Board Opinion (APB)

-Number 21

-Paragraph 16, 20

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 1A

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28541-108399

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-Publisher FASB

-Name FASB Staff Position (FSP)

-Number APB14-1

-Paragraph 31

-Subparagraph b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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- Definition

Amount borrowed under the credit facility as of the balance sheet date.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

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Balance Type:	credit
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- Definition

Decrease for amounts repaid on the credit facility for the period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(f))

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-Name Regulation S-X (SX)

-Number 210

-Section 08

-Paragraph f

-Article 4

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- Definition

Date the credit facility terminates, in CCYY-MM-DD format.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(b),22(b))

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-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

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- Definition

The effective interest rate at the end of the reporting period.

+ References

+ Details

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- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 129

-Paragraph 2, 4

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(b),22(b))

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

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