Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Summary of bond activity
Fair value at beginning of period	$ 29,776	$ 30,923
Purchases	975	5,742
Sales	(345)	(2,000)
Maturities and calls	(3,000)	(4,900)
Decrease in bond premium	(216)	(290)
Fair market value adjustment	65	301
Fair value at end of period	$ 27,255	$ 29,776