Investments in Associated Companies - Effect of Change in Accounting Principle (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended							12 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2011	Dec. 31, 2010
Income Statement [Abstract]
Effect of Change in Accounting Principle on Equity in Net Income of Associated Companies	$ 439	$ 568	$ 419	$ 600	$ 584	$ 713	$ 426	$ 2,323	$ 313
Impact of Change in Accounting Method on Net Income	439	568	419	600	584	713	426	2,323	313
Impact of Change in Accounting Method on Earnings Per Share, Diluted	$ 0.03	$ 0.04	$ 0.04	$ 0.05	$ 0.04	$ 0.06	$ 0.03	$ 0.19	$ 0.03
Statement Of Financial Position [Abstract]
Effect of Change in Accounting Principle on Investments in Associated Companies	8,685	7,819	7,620	6,131	4,919	5,248	4,486	6,131	3,938
Effect of Change in Accounting Principle on Other Assets	(500)	(500)	(500)	(500)	(500)	(500)	(500)	(500)	(500)
Effect of Change in Accounting Principle on Total Assets	8,185	7,319	7,120	5,631	4,419	4,748	3,986	5,631	3,438
Equity [Abstract]
Effect of Change in Accounting Principle on Retained Earnings	6,204	5,765	5,197	4,778	4,178	3,594	2,881	4,778	2,455
Effect of Change in Accounting Principle on Accumulated Other Comprehensive Loss	1,981	1,554	1,923	853	241	1,154	1,105	853	983
Effect of Change in Accounting Principle on Total Equity	$ 8,185	$ 7,319	$ 7,120	$ 5,631	$ 4,419	$ 4,748	$ 3,986	$ 5,631	$ 3,438