Accumulated Comprehensive Margin (Deficit) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Changes in Accumulated Other Comprehensive Margin (Deficit) of Available-for-sale Securities
Balance at the beginning of the period	$ 1,327	$ (490)	$ 618	$ (469)
Unrealized gain	119	613	828	592
Balance at the end of the period	$ 1,446	$ 123	$ 1,446	$ 123