Subsequent Events (Narrative) (Details)							1 Months Ended	3 Months Ended							4 Months Ended
	Oct. 15, 2018 $ / shares	Oct. 05, 2018 a	Oct. 04, 2018 $ / shares	Sep. 20, 2018	Jun. 25, 2018	Mar. 15, 2018	Apr. 14, 2018 $ / shares	Dec. 31, 2018 $ / shares	Sep. 30, 2018 $ / shares	Jun. 30, 2018 $ / shares	Mar. 31, 2018 $ / shares	Sep. 30, 2017 $ / shares	Jun. 30, 2017 $ / shares	Mar. 31, 2017 $ / shares	Oct. 14, 2018 $ / shares
Subsequent Event [Line Items]
Dividends payable, date payable												Jul. 06, 2017	Apr. 05, 2017	Jan. 05, 2017
Dividend paid to common stockholders												$ 0.39	$ 0.39	$ 0.36
Dividends payable, date of record				Sep. 20, 2018								Jun. 16, 2017	Mar. 16, 2017	Dec. 16, 2016
Series A Preferred Stock
Subsequent Event [Line Items]
Preferred stock dividends per share cash paid							$ 0.14844
Dividend rate (as a percent)						7.125%
Series B Preferred Stock
Subsequent Event [Line Items]
Dividend rate (as a percent)					6.50%
Common stock
Subsequent Event [Line Items]
Dividends payable, date payable									Jul. 06, 2018	Apr. 05, 2018	Jan. 05, 2018
Dividend paid to common stockholders									$ 0.41	$ 0.41	$ 0.39
Partnership distribution per unit								$ 0.39	$ 0.41
Dividends payable, date of record									Jun. 20, 2018	Mar. 22, 2018	Dec. 05, 2017
Preferred stock
Subsequent Event [Line Items]
Dividends payable, date payable									Jul. 16, 2018	Apr. 16, 2018
Dividend paid to common stockholders									$ 0.45	$ 0.15
Partnership distribution per unit									$ 0.15
Dividends payable, date of record									Jun. 29, 2018	Apr. 05, 2018
Subsequent Event
Subsequent Event [Line Items]
The number of acres of land acquired | a		55
Subsequent Event | Series A Preferred Stock
Subsequent Event [Line Items]
Dividends payable, date payable	Oct. 15, 2018
Preferred stock dividends per share cash paid	$ 0.45
Dividend rate (as a percent)	7.125%
Subsequent Event | Series B Preferred Stock
Subsequent Event [Line Items]
Dividends payable, date payable	Oct. 15, 2018
Preferred stock dividends per share cash paid															$ 1.9861111
Dividend rate (as a percent)	6.50%
Subsequent Event | Common stock
Subsequent Event [Line Items]
Dividends payable, date payable			Oct. 04, 2018
Dividend paid to common stockholders			$ 0.41
Partnership distribution per unit			$ 0.41