Equity (Preferred Stock) (Details) - USD ($)	1 Months Ended	12 Months Ended
	Oct. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Oct. 20, 2017	Dec. 31, 2016
Preferred Stock
Shares Issued and Outstanding		16,200,000	16,200,000
Carrying Value		$ 476,000,000	$ 476,000,000
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
Capital gains distribution percentage		100.00%	100.00%
Unrecaptured Section 1250 gain percentage		26.02%	27.90%
Long term capital gain percentage		73.98%	72.10%
Amount of preferred dividends in arrears		$ 0
Maximum
Preferred Stock
Number of days prior to dividend payment date that Board of Directors may elect to designate as the payment date		30 days
Minimum
Preferred Stock
Number of days prior to dividend payment date that Board of Directors may elect to designate as the payment date		10 days
Series D
Preferred Stock
Shares Issued and Outstanding		4,000,000	4,000,000
Par Value (in dollars per share)		$ 0.001	$ 0.001
Liquidation Preference (in dollars per share)		$ 25.00	$ 25
Rate per Annum (as a percent)		8.00%	8.00%
Equivalent to Fixed Annual Rate (in dollars per share)		$ 2.00	$ 2.00
Carrying Value		$ 89,041,000	$ 89,041,000
Dividends declared and paid		$ 8,000,000	$ 8,000,000
Series G
Preferred Stock
Shares Issued and Outstanding		3,200,000	3,200,000
Par Value (in dollars per share)		$ 0.001	$ 0.001
Liquidation Preference (in dollars per share)		$ 25.00	$ 25
Rate per Annum (as a percent)		7.65%	7.65%
Equivalent to Fixed Annual Rate (in dollars per share)		$ 1.91	$ 1.91
Carrying Value		$ 72,664,000	$ 72,664,000
Dividends declared and paid		$ 6,100,000	$ 6,100,000
Series I
Preferred Stock
Shares Issued and Outstanding		5,000,000	5,000,000
Par Value (in dollars per share)		$ 0.001	$ 0.001
Liquidation Preference (in dollars per share)		$ 25.00	$ 25
Rate per Annum (as a percent)		7.50%	7.50%
Equivalent to Fixed Annual Rate (in dollars per share)		$ 1.88	$ 1.88
Carrying Value		$ 120,785,000	$ 120,785,000
Dividends declared and paid		$ 9,400,000	$ 9,400,000
Series J convertible perpetual preferred stock
Preferred Stock
Shares Issued and Outstanding		4,000,000	4,000,000
Par Value (in dollars per share)		$ 0.001	$ 0.001
Liquidation Preference (in dollars per share)		$ 50	$ 50
Rate per Annum (as a percent)		4.50%	4.50%
Equivalent to Fixed Annual Rate (in dollars per share)		$ 2.25	$ 2.25
Carrying Value		$ 193,510,000	$ 193,510,000
Dividends declared and paid		$ 9,000,000	9,000,000
Redemption price as a percentage of liquidation preference		100.00%
Shares issued upon conversion		3.9762
Conversion price (in dollars per share)		$ 12.57
Series E and F Preferred Stock
Preferred Stock
Liquidation Preference (in dollars per share)				$ 25.00
Carrying Value					$ 223,700,000
Liquidation preference, value				$ 240,000,000
Redemption premium	$ 16,300,000
Series D, E, F, G and I Preferred Stock
Preferred Stock
Liquidation Preference (in dollars per share)					$ 25.00
Series E
Preferred Stock
Dividends declared and paid	1,100,000		8,300,000
Series F
Preferred Stock
Dividends declared and paid	$ 800,000		$ 5,900,000
Series G and I Preferred Stock
Preferred Stock
Liquidation Preference (in dollars per share)		$ 25.00
Redemption price as a percentage of liquidation preference		100.00%