Financial Instruments and Concentration of Risk (Estimated Fair Values of Financial Instruments) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and short-term investments, Carrying Amount	$ 57,837	$ 80,062
Cash and short-term investments, Fair Value	57,837	80,062
Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current installments on lont-term debt and fair value adjustments related to fair value swaps), Carrying Amount	320,092	357,576
Long-term debt (including current installments on lont-term debt and fair value adjustments related to fair value swaps), Fair Value	347,869	379,048
Level 2 [Member] | Interest rate contracts [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net derivative asset related to interest rate swaps, Carrying Amount	1,830	2,952
Net derivative asset related to interest rate swaps, Fair Value	$ 1,830	$ 2,952