Accumulated Comprehensive Margin (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Accumulated other comprehensive margin (deficit)
Balance at the beginning of the period	$ 691	$ 1,327	$ 903	$ 618
Unrealized gain (loss)	(1,074)	119	(1,222)	911
(Gain) reclassified to net margin	(16)		(80)	(83)
Balance at the end of the period	$ (399)	$ 1,446	$ (399)	$ 1,446
Effective income tax rate (as a percent)			0.00%