Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Summary of bond activity
Fair value at January 1	$ 30,923	$ 15,255
Purchases	5,742	19,643
Sales	(2,000)	(3,000)
Maturities and calls	(4,900)	(1,916)
(Decrease) increase in bond premium	(290)	914
Fair market value adjustment	301	27
Fair value at December 31	$ 29,776	$ 30,923