Equity Offerings - Preferred Share Offerings (Details) - USD ($) $ / shares in Units, $ in Millions			1 Months Ended		12 Months Ended
	Oct. 12, 2017	Sep. 11, 2017	Oct. 04, 2017	Jan. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Oct. 13, 2017	Jan. 27, 2017
Class Of Stock [Line Items]
Convertible Preferred Stock, Settlement Terms					We may not redeem the Series C Preferred Shares and the Series D Preferred Shares before January 27, 2022 and September 15, 2022, respectively, except to preserve our status as a REIT or upon the occurrence of a Change of Control, as defined in the Trust Agreement addendums designating the Series C Preferred Shares and Series D Preferred Shares. On and after January 27, 2022 for the Series C Preferred Shares and September 15, 2022 for the Series D Preferred Shares, we may redeem any or all of the Series C Preferred Shares or Series D Preferred Shares at $25.00 per share plus any accrued and unpaid dividends. In addition, upon the occurrence of a Change of Control, we may redeem any or all of the Series C Preferred Shares or Series D Preferred Shares for cash within 120 days after the first date on which such Change of Control occurred, at $25.00 per share plus any accrued and unpaid dividends. The Series C Preferred Shares and Series D Preferred Shares have no stated maturity, are not subject to any sinking fund or mandatory redemption provisions, and will remain outstanding indefinitely unless we redeem or otherwise repurchase them or they are converted.
Redemption price per share							$ 25.00
Series C Preferred Stock
Class Of Stock [Line Items]
Preferred Stock, Shares Issued					6,900,000	6,900,000		6,900,000
Shares Issued, Price Per Share								$ 25.00
Proceeds from Issuance or Sale of Equity				$ 166.3
Underwriting Discount On Public Offering								$ 5.4
Underwriting Discount Per Share On Public Offering								$ 0.7875
Equity offering expenses								$ 0.8
Repayments of Debt				$ 117.0
Preferred shares dividend rate				7.20%
Series D Preferred Stock
Class Of Stock [Line Items]
Preferred Stock, Shares Issued		5,000,000			5,000,000	5,000,000
Shares Issued, Price Per Share		$ 25.00
Proceeds from Issuance or Sale of Equity			$ 120.5
Underwriting Discount On Public Offering		$ 4.0
Underwriting Discount Per Share On Public Offering		$ 0.7875
Equity offering expenses		$ 0.5
Preferred shares dividend rate		6.875%
Series D Preferred Stock | Over-Allotment Option
Class Of Stock [Line Items]
Stock Issued During Period, Shares, New Issues		200,000
Series A Preferred Stock
Class Of Stock [Line Items]
Preferred shares dividend rate	8.25%
Redemption price per share	$ 25.00