Equity (Details) (USD $) In Millions, except Share data in Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013		Mar. 31, 2013	Dec. 31, 2012
Class of Stock [Line Items]
Shares Issued and Outstanding (in thousands)	25,800
Number of days in year used in the computation of preferred stock dividends for any partial dividend period	360 days
Number of months used in the computation of preferred stock dividends for any partial dividend period	12 months
Number of days in month, dividends computation of dividends payable for any partial dividend period	30 days
Amount of preferred dividends in arrears	$ 0
Maximum
Class of Stock [Line Items]
Number of days prior to dividend payment date that Board of Directors may elect to designate as the payment date	30 days
Minimum
Class of Stock [Line Items]
Number of days prior to dividend payment date that Board of Directors may elect to designate as the payment date	10 days
Series D
Class of Stock [Line Items]
Shares Issued and Outstanding (in thousands)	4,000
Par Value (in dollars per share)	$ 0.001
Liquidation Preference (in dollars per share)	$ 25
Rate per Annum	8.00%	[1],[2]
Equivalent to Fixed Annual Rate (in dollars per share)	$ 2.00	[1],[2]
Dividends declared and paid	4.0
Series E
Class of Stock [Line Items]
Shares Issued and Outstanding (in thousands)	5,600
Par Value (in dollars per share)	$ 0.001
Liquidation Preference (in dollars per share)	$ 25
Rate per Annum	7.875%	[1],[2]
Equivalent to Fixed Annual Rate (in dollars per share)	$ 1.97	[1],[2]
Dividends declared and paid	5.5
Series F
Class of Stock [Line Items]
Shares Issued and Outstanding (in thousands)	4,000
Par Value (in dollars per share)	$ 0.001
Liquidation Preference (in dollars per share)	$ 25
Rate per Annum	7.80%	[1],[2]
Equivalent to Fixed Annual Rate (in dollars per share)	$ 1.95	[1],[2]
Dividends declared and paid	3.9
Series G
Class of Stock [Line Items]
Shares Issued and Outstanding (in thousands)	3,200
Par Value (in dollars per share)	$ 0.001
Liquidation Preference (in dollars per share)	$ 25
Rate per Annum	7.65%	[1],[2]
Equivalent to Fixed Annual Rate (in dollars per share)	$ 1.91	[1],[2]
Dividends declared and paid	3.1
Series I
Class of Stock [Line Items]
Shares Issued and Outstanding (in thousands)	5,000
Par Value (in dollars per share)	$ 0.001
Liquidation Preference (in dollars per share)	$ 25
Rate per Annum	7.50%	[1],[2]
Equivalent to Fixed Annual Rate (in dollars per share)	$ 1.88	[1],[2]
Dividends declared and paid	4.7
Series J
Class of Stock [Line Items]
Shares Issued and Outstanding (in thousands)	4,000
Par Value (in dollars per share)	$ 0.001
Liquidation Preference (in dollars per share)	$ 50		$ 50.00	$ 0
Rate per Annum	4.50%	[1],[2]
Equivalent to Fixed Annual Rate (in dollars per share)	$ 2.25	[1],[2]
Dividends declared and paid	$ 2.2

[1] The Company declared and paid dividends of $4.0 million, $5.5 million, $3.9 million, $3.1 million, and $4.7 million on its Series D, E, F, G, and I preferred stock, respectively, during each of the six months ended June 30, 2013 and 2012. The Company also declared and paid dividends of $2.2 million on its Series J preferred stock during the six months ended June 30, 2013. All of the dividends qualified as return of capital for tax reporting purposes. There are no dividend arrearages on any of the preferred shares currently outstanding.

[2] Holders of shares of the Series D, E, F, G, I and J preferred stock are entitled to receive dividends, when and as declared by the Board of Directors, out of funds legally available for the payment of dividends. Dividends are cumulative from the date of original issue and are payable quarterly in arrears on or before the 15th day of each March, June, September and December or, if not a business day, the next succeeding business day. Any dividend payable on the preferred stock for any partial dividend period will be computed on the basis of a 360-day year consisting of twelve 30-day months. Dividends will be payable to holders of record as of the close of business on the first day of the calendar month in which the applicable dividend payment date falls or on another date designated by the Board of Directors of the Company for the payment of dividends that is not more than 30 nor less than 10 days prior to the dividend payment date.