Preferred Stock Offerings (Details)									12 Months Ended				24 Months Ended
	May 06, 2021 USD ($) $ / shares shares	Apr. 15, 2021 USD ($) shares	Mar. 23, 2021 USD ($) $ / shares shares	Sep. 23, 2019 USD ($) $ / shares shares	Aug. 19, 2019 USD ($) $ / shares shares	Jun. 27, 2019 USD ($) $ / shares shares	Apr. 12, 2019 USD ($) shares	Mar. 28, 2019 USD ($) $ / shares shares	Dec. 31, 2021 USD ($) $ / shares	Dec. 31, 2020 $ / shares	Dec. 31, 2019 USD ($)	Dec. 31, 2016 $ / shares	Dec. 31, 2016 $ / shares shares	Dec. 31, 2015 $ / shares
Public Offering of Preferred Stock
Redemption of 8% preferred stock		$ 41,600,000
Net proceeds									$ 191,084,000		$ 192,915,000
8% Preferred Stock
Public Offering of Preferred Stock
Redemption of 8% preferred stock (in shares) | shares		41,625
Redemption of 8% preferred stock		$ 41,600,000
Preferred stock, dividend rate (as a percent)	8.00%	8.00%							8.00%	8.00%
Preferred stock liquidation preference (in dollars per share) | $ / shares									$ 1,000	$ 1,000
Final dividend for redemption of 8% preferred stock		$ 139,000
8% Preferred Stock | Private Placement
Public Offering of Preferred Stock
Shares issued (in shares) | shares													41,625
Preferred stock, dividend rate (as a percent)												8.00%
Preferred stock liquidation preference (in dollars per share) | $ / shares												$ 1,000.00	$ 1,000.00	$ 1,000.00
7% Preferred Stock
Public Offering of Preferred Stock
Redemption of 8% preferred stock (in shares) | shares				874,000
Preferred stock, dividend rate (as a percent)				7.00%					7.00%	7.00%
Preferred stock liquidation preference (in dollars per share) | $ / shares				$ 25					$ 25	$ 25
Redemption of preferred stock				$ 21,850,000
Brokerage fees				$ 0
7% Preferred Stock | Public Offering
Public Offering of Preferred Stock
Shares issued (in shares) | shares								2,000,000
Preferred stock, dividend rate (as a percent)								7.00%
Preferred stock liquidation preference (in dollars per share) | $ / shares								$ 25.00
Aggregate gross offering proceeds for the shares issued								$ 50,000,000.0
Net proceeds								48,300,000
Offering costs								$ 1,700,000
7% Preferred Stock | Private Placement
Public Offering of Preferred Stock
Shares issued (in shares) | shares						874,000
Preferred stock, dividend rate (as a percent)						7.00%
Preferred stock liquidation preference (in dollars per share) | $ / shares						$ 25.00
Aggregate gross offering proceeds for the shares issued						$ 21,850,000
6% Series B Preferred Stock
Public Offering of Preferred Stock
Preferred stock, dividend rate (as a percent)									6.00%	6.00%
Preferred stock liquidation preference (in dollars per share) | $ / shares									$ 1,000	$ 1,000
6% Series B Preferred Stock | Public Offering
Public Offering of Preferred Stock
Depositary shares issued (in shares) | shares					5,000,000
Depositary shares equivalent preferred stock interest per share					0.025
Preferred stock, dividend rate (as a percent)					6.00%
Preferred stock liquidation preference (in dollars per share) | $ / shares					$ 1,000.00
Depositary share, preferred stock liquidation preference (in dollars per share) | $ / shares					$ 25.00
Net proceeds					$ 120,800,000
Offering costs					125,000,000.0
Underwriting discounts					$ 4,200,000
6% Series C Preferred Stock
Public Offering of Preferred Stock
Shares issued (in shares) | shares	46,181
Depositary shares issued (in shares) | shares	1,847,233
Depositary share price (in dollars per share) | $ / shares	$ 25
Preferred stock, dividend rate (as a percent)									6.00%
Preferred stock liquidation preference (in dollars per share) | $ / shares									$ 1,000
Net proceeds	$ 46,200,000
Offering costs	$ 23,000
6% Series C Preferred Stock | Public Offering
Public Offering of Preferred Stock
Depositary shares issued (in shares) | shares			6,000,000
Preferred stock, dividend rate (as a percent)			6.00%
Preferred stock liquidation preference (in dollars per share) | $ / shares			$ 1,000.00
Depositary share, preferred stock liquidation preference (in dollars per share) | $ / shares			$ 25.00
Net proceeds			$ 144,900,000
Offering costs			150,000,000.0
Underwriting discounts			$ 5,100,000
Series A Preferred Stock | Public Offering
Public Offering of Preferred Stock
Shares issued (in shares) | shares							81,800
Net proceeds							$ 2,000,000.0
Underwriting discounts							$ 41,000