Consolidated Statements of Comprehensive Margin (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Consolidated Statements of Comprehensive Margin
Net Margin	$ (25,150)	$ 20,104	$ 24,502	$ 22,024	$ (8,821)	$ 23,743	$ 10,878	$ 13,520	$ 41,480	$ 39,320	$ 37,737
Other comprehensive margin:
Unrealized (loss) gain on available-for-sale securities									(1,452)	285	1,087
Total comprehensive margin									$ 40,028	$ 39,605	$ 38,824