Debt (Details Textual) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Current Unsecured Bank Credit Facility [Member]
Debt (Textual) [Abstract]
Line of Credit Facility, Expiration Date	Jul. 30, 2019
Senior Notes 2015 [Member] \| Senior Note 6.05 % [Member]
Debt (Textual) [Abstract]
Unaccrued discount	$ 5	$ 11
Senior Notes	6.05%
Aggregate principal amount	75,000	75,000
Maturity date of Senior note	Jun. 15, 2015	Jun. 15, 2015
Imputed rate	6.10%	6.10%
Senior Notes 2016 [Member] \| Senior Note 6.85 % [Member]
Debt (Textual) [Abstract]
Unaccrued discount	40	50
Senior Notes	6.85%
Aggregate principal amount	$ 125,000	$ 125,000
Maturity date of Senior note	Apr. 15, 2016	Apr. 15, 2016
Imputed rate	6.90%	6.90%

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Face (par) amount of debt instrument at time of issuance.

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-Publisher FASB

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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.

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

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- Definition

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

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

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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.

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- Definition

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

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