Consolidated Statements of Comprehensive Margin (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Consolidated Statements of Comprehensive Margin
Net Margin	$ (8,821)	$ 23,743	$ 10,878	$ 13,520	$ (1,499)	$ 10,408	$ 12,718	$ 16,110	$ 39,320	$ 37,737	$ 33,733
Other comprehensive margin:
Unrealized gain on available-for-sale securities									285	1,087	784
Total comprehensive margin									$ 39,605	$ 38,824	$ 34,517