Subsequent Events (Details) (USD $)	9 Months Ended		1 Months Ended			9 Months Ended	0 Months Ended			1 Months Ended
	Sep. 30, 2013	Sep. 30, 2013 Credit Facility	Mar. 31, 2013 Common stock	Apr. 30, 2012 Class F preferred stockholders	Feb. 29, 2012 Class F preferred stockholders	Sep. 30, 2012 Class F preferred stockholders	Oct. 24, 2013 Subsequent event Maryland State Action	Oct. 31, 2013 Subsequent event Credit Facility	Oct. 29, 2013 Subsequent event Credit Facility	Oct. 31, 2013 Subsequent event Common stock	Oct. 31, 2013 Subsequent event Class E preferred stockholders	Oct. 31, 2013 Subsequent event Class F preferred stockholders
Subsequent events
Common stock, dividends declared (in dollars per share)										$ 0.1818542
Preferred stock, dividends declared (in dollars per share)											$ 0.140625	$ 0.138021
Shares issued			17,250,000	1,400,000	14,950,000					9,775,000
Issuance price (in dollars per share)			$ 45.90							$ 40.63
Common stock purchased by the underwriters (in shares)			2,250,000							1,275,000
Underwriting discounts and offering costs			$ 36,700,000			$ 13,800,000				$ 18,800,000
Net proceeds from issuance of common shares	755,085,000		755,100,000							378,400,000
Increase in the maximum borrowing capacity after amendment								500,000,000	500,000,000
Maximum borrowing capacity		1,000,000,000						1,500,000,000	1,500,000,000
Attorneys' fees and expenses to be paid							$ 512,751
Percentage of attorneys' fees and expenses to be paid							25.00%