Debt (Details Textual) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Debt (Textual) [Abstract]
Offer and sale of various securities	$ 300,000
Debt Instrument, Basis Spread on Variable Rate	1.25%
Previous Unsecured Bank Credit Facility [Member]
Debt (Textual) [Abstract]
Expiry date for agreement	Dec. 19, 2011
Repayment of balance outstanding	38,000
Current Unsecured Bank Credit Facility [Member]
Debt (Textual) [Abstract]
Unsecured borrowings	150,000
Expiry date for agreement	Oct. 06, 2015
Effective interest rate	1.49%
Current borrowing under the Current Bank Credit Facility	38,000
Variable commitment fee unused portion	0.15%
Senior Notes 2015 [Member]
Debt (Textual) [Abstract]
Unaccrued discount	38	65
Senior Notes	6.05%
Aggregate principal amount	75,000
Maturity date of Senior note	Jun. 15, 2015
Discount on issue	0.357%
Effective yield	6.098%
Percentage of principal amount of senior notes for redemption	100.00%
Treasury yield basis point	30 basis points
Senior Notes 2016 [Member]
Debt (Textual) [Abstract]
Unaccrued discount	88	126
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	30 basis points