Dividends on Common Stock (Details) (Unaudited) (USD $)	3 Months Ended				9 Months Ended
	Jun. 30, 2011	Mar. 31, 2011	Jun. 30, 2010	Mar. 31, 2010	Sep. 30, 2011	Sep. 30, 2010
Common Stock Dividends Numeric [Abstract]
Quarterly common stock dividend	$ 0.50	$ 0.47	$ 0.47	$ 0.43	$ 1.47	$ 1.37