Stockholders' Equity - Narrative (Details) - USD ($)	1 Months Ended		3 Months Ended				9 Months Ended		12 Months Ended
	Apr. 30, 2020	Mar. 31, 2020	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Apr. 20, 2020	Mar. 31, 2018
Conversion of Stock [Line Items]
Common stock, outstanding (in shares)			89,614,601				89,614,601		89,458,752
Common stock, issued (in shares)			89,614,601		89,458,752		89,614,601	89,458,752	89,458,752
Preferred stock, authorized (in shares)			30,000,000				30,000,000
Preferred stock, outstanding (in shares)			6,799,467				6,799,467		6,799,467
Common stock dividend rate (in dollars per share)		$ 1.60		$ 2.13
Common stock, monthly dividend rate (in dollars per share)		$ 0.40		$ 0.5325
Agent | At-the-Market Program
Conversion of Stock [Line Items]
Number of shares issued (in shares)			0		5,596,452	7,759,322	0	13,555,774
Gross proceeds from sale of stock					$ 109,900,000	$ 152,800,000		$ 262,600,000
Commissions paid					1,600,000	1,500,000		3,100,000
Payments of stock offering costs					$ 200,000	800,000		$ 1,000,000.0
Aggregate common stock offering price						$ 175,000,000.0
Series A Preferred Stock
Conversion of Stock [Line Items]
Payments of stock offering costs							$ 75,000
Preferred stock, authorized (in shares)			9,959,650				9,959,650		9,959,650
Preferred stock, dividend rate							7.25%		7.25%
Preferred stock, par value (in dollars per share)			$ 0.01				$ 0.01		$ 0.01
Preferred stock, issued (in shares)			6,799,467				6,799,467		6,799,467
Preferred stock, outstanding (in shares)			6,799,467				6,799,467		6,799,467
Dividend payout (in dollars per share)							$ 0.453125
Preferred stock, liquidation preference (in dollars per share)			$ 25.00				$ 25.00		$ 25.00
Series A Preferred Stock | Agent | At-the-Market Program
Conversion of Stock [Line Items]
Number of shares issued (in shares)					724,600			1,265,558
Gross proceeds from sale of stock					$ 18,500,000			$ 32,300,000
Commissions paid					300,000			500,000
Payments of stock offering costs					$ 100,000			$ 200,000
Aggregate preferred stock offering price											$ 200,000,000.0
Series B Preferred Stock
Conversion of Stock [Line Items]
Preferred stock, authorized (in shares)			11,450,000				11,450,000		11,450,000
Preferred stock, dividend rate							6.875%		6.875%
Preferred stock, par value (in dollars per share)			$ 0.01				$ 0.01		$ 0.01
Preferred stock, issued (in shares)			3,635,128				3,635,128		3,450,000
Preferred stock, outstanding (in shares)			3,635,128				3,635,128		3,450,000
Preferred stock, liquidation preference (in dollars per share)			$ 25.00				$ 25.00		$ 25.00
Preferred stock, quarterly dividend (in dollars per share)							0.429688
Series B Preferred Stock | Public Offering
Conversion of Stock [Line Items]
Preferred stock, liquidation preference (in dollars per share)			$ 25.00				$ 25.00
Series B Preferred Stock | Agent | At-the-Market Program
Conversion of Stock [Line Items]
Number of shares issued (in shares)			185,128				185,128
Gross proceeds from sale of stock			$ 4,600,000				$ 4,600,000
Commissions paid			70,000				70,000
Payments of stock offering costs			$ 100,000				$ 100,000
Aggregate preferred stock offering price									$ 200,000,000
Series C Preferred Stock
Conversion of Stock [Line Items]
Preferred stock, authorized (in shares)			100,000				100,000		0
Preferred stock, par value (in dollars per share)			$ 0.01				$ 0.01
Preferred stock, issued (in shares)			0				0		0
Preferred stock, outstanding (in shares)			0				0		0
Percent of outstanding shares acquired	4.90%
Exercise price (in dollars per share)	$ 50.00
Preferred share purchase right basis per share										0.001