Preferred Stock (Details)							12 Months Ended				24 Months Ended
	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	Nov. 02, 2017 USD ($)	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 $ / shares	Dec. 31, 2016 $ / shares	Dec. 31, 2015 $ / shares	Dec. 31, 2016 $ / shares shares
Public Offering of Preferred Stock
Offering costs						$ 8,800,000
Net proceeds							$ 192,915,000
8% Preferred Stock
Public Offering of Preferred Stock
Preferred stock, dividend rate (as a percent)							8.00%	8.00%
Preferred stock liquidation preference (in dollars per share) | $ / shares							$ 1,000	$ 1,000
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%	8.00%
Preferred stock liquidation preference (in dollars per share) | $ / shares									$ 1,000.00	$ 1,000.00	$ 1,000.00
Series A preferred stock | Public Offering
Public Offering of Preferred Stock
Shares issued (in shares) | shares				81,800
Net proceeds				$ 2,000,000
Underwriting discounts				$ 41,000
7% Preferred Stock
Public Offering of Preferred Stock
Shares redeemed (in shares) | shares	874,000
Preferred stock, dividend rate (as a percent)	7.00%						7.00%
Preferred stock liquidation preference (in dollars per share) | $ / shares	$ 25						$ 25
Aggregate cost	$ 21,850,000
Offering costs							$ 1,824,000
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
Offering costs					1,700,000
Net proceeds					$ 48,300,000
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% Preferred Stock
Public Offering of Preferred Stock
Preferred stock, dividend rate (as a percent)							6.00%
Preferred stock liquidation preference (in dollars per share) | $ / shares							$ 1,000
Offering costs							$ 4,156,000
6% 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
Offering costs		$ 125,000,000
Net proceeds		120,800,000
Underwriting discounts		$ 4,200,000
Preferred Stock | 7% Preferred Stock
Public Offering of Preferred Stock
Shares issued (in shares) | shares							2,955,800
Preferred Stock | 6% Preferred Stock
Public Offering of Preferred Stock
Shares issued (in shares) | shares							125,000