Basis of Presentation - Offering of Preferred Stock (Details)							3 Months Ended			9 Months Ended	12 Months Ended
	Sep. 23, 2019 USD ($)	Aug. 19, 2019 USD ($) $ / shares shares	Jun. 27, 2019 USD ($) $ / shares shares	Apr. 12, 2019 USD ($) shares	Mar. 28, 2019 USD ($) $ / shares shares	Mar. 23, 2019 USD ($) $ / shares shares	Sep. 30, 2019 USD ($) $ / shares	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 2019 USD ($) $ / shares	Dec. 31, 2018 $ / shares
Public Offering of Preferred Stock
Net proceeds										$ 192,958,000
Series A preferred stock
Public Offering of Preferred Stock
Shares issued (in shares) | shares				81,800
Net proceeds				$ 2,000,000
Underwriting discounts				$ 41,000
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	$ 1,000
7% Preferred Stock
Public Offering of Preferred Stock
Shares issued (in shares) | shares					2,000,000
Shares redeemed (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					$ 25.00	$ 25	25			$ 25
Aggregate gross offering proceeds for the shares issued					$ 50,000,000
Aggregate cost						$ 21,850,000
Offering costs					1,700,000			$ 69,000	$ 1,731,000
Net proceeds					$ 48,300,000
Brokerage fees	$ 0
7% Preferred Stock | Private Placement Offering
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
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%								6.00%
Preferred stock liquidation preference (in dollars per share) | $ / shares		$ 1,000.00					$ 1,000			$ 1,000
Depositary share, preferred stock liquidation preference (in dollars per share) | $ / shares		$ 25.00
Offering costs		$ 125,000,000					$ 4.138
Net proceeds		120,900,000
Underwriting discounts		$ 4,100,000