Subsequent Events (Details) (Subsequent event, USD $)	1 Months Ended
	Apr. 30, 2013
Common stock
Subsequent events
Common stock, dividends declared (in dollars per share)	$ 0.1812292
Class E preferred stockholders
Subsequent events
Preferred stock, dividends declared (in dollars per share)	$ 0.140625
Class F preferred stockholders
Subsequent events
Preferred stock, dividends declared (in dollars per share)	$ 0.138021