Debt (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Short-term debt:
Bank Credit Facility, expires October 6, 2015	$ 38,000	$ 38,000
Long-term debt:
Long-term debt	199,842	199,809
Total	237,842	237,809
Senior Notes 2015 [Member]
Short-term debt:
Bank Credit Facility, expires October 6, 2015	38,000	38,000
Senior Notes 2015 [Member] | Senior Note 6.05 % [Member]
Long-term debt:
Long-term debt	74,949	74,935
Senior Notes 2016 [Member] | Senior Note 6.85 % [Member]
Long-term debt:
Long-term debt	$ 124,893	$ 124,874