Intangible Assets (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Summary of amortizable intangible assets
Amortizing intangible assets, Gross Carrying Amount	$ 31,999	$ 31,999
Amortizing intangible assets, Accumulated amortization	(26,451)	(22,970)
Aggregate Amortization Expense:
Aggregate Amortization Expense, Gross Carrying Amount	3,481	3,732	3,163
Estimated Amortization Expense:
Estimated Amortization Expense, 2012	2,159
Estimated Amortization Expense, 2013	1,127
Estimated Amortization Expense, 2014	475
Estimated Amortization Expense, 2015	437
Estimated Amortization Expense, 2016	395
Estimated Amortization Expense, Thereafter	$ 955