Subsequent Events (Details) (USD $) In Thousands, except Share data, unless otherwise specified	6 Months Ended		6 Months Ended	12 Months Ended	0 Months Ended		0 Months Ended	1 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2013 Restricted stock	Dec. 31, 2012 Restricted stock	Jul. 03, 2013 Subsequent event 4.65% notes, issued in July 2013 and due in August 2023	Jul. 31, 2013 Subsequent event 4.65% notes, issued in July 2013 and due in August 2023	Jul. 03, 2013 Subsequent event Restricted stock	Jul. 31, 2013 Subsequent event Common stock	Jul. 31, 2013 Subsequent event Class E preferred stockholders	Jul. 31, 2013 Subsequent event Class F preferred stockholders
Subsequent events
Common stock, dividends declared (in dollars per share)								$ 0.1815417
Preferred stock, dividends declared (in dollars per share)									$ 0.140625	$ 0.138021
Vesting period			10 years				5 years
Number of shares of restricted stock vested			324,625	290,877			212,827
Notes payable	$ 2,450,000	$ 2,550,000				$ 750,000
Interest rate (as a percent)					4.65%
Percentage price paid to the investor					99.775%
Effective yield (as a percent)					4.678%
Proceeds from term loan	$ 70,000				$ 742,700