Subsequent Events (Details) (USD $)	3 Months Ended	1 Months Ended
	Mar. 31, 2013	Mar. 31, 2013 Common stock	Apr. 30, 2014 Subsequent event Common stock	Apr. 30, 2014 Subsequent event Class E preferred stockholders	Apr. 30, 2014 Subsequent event Class F preferred stockholders
Subsequent events
Common stock, dividends declared (in dollars per share)			$ 0.1824792
Preferred stock, dividends declared (in dollars per share)				$ 0.140625	$ 0.138021
Shares issued		17,250,000	13,800,000
Issuance price (in dollars per share)		$ 45.90	$ 39.96
Common stock purchased by the underwriters (in shares)		2,250,000	1,800,000
Underwriting discounts and offering costs		$ 36,300,000	$ 23,000,000
Proceeds from common stock offerings, net	$ 755,495,000	$ 755,500,000	$ 528,500,000