Dividends on Common Stock (Details) (USD $)	3 Months Ended						12 Months Ended
	Jun. 30, 2011	Mar. 31, 2011	Jun. 30, 2010	Mar. 31, 2010	Jun. 30, 2009	Mar. 31, 2009	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Dividends on Common Stock (Amounts in Paragraphs)
Quarterly common stock dividend	$ 0.50	$ 0.47	$ 0.47	$ 0.43	$ 0.43	$ 0.40	$ 1.97	$ 1.84	$ 1.69