Subsequent Events - Real Estate (Details) - USD ($)	1 Months Ended	3 Months Ended		6 Months Ended
	Jul. 31, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Real Estate Investments
Payments on debt				$ 4,166,000	$ 4,167,000
Outstanding Balance		$ 652,440,000		652,440,000		$ 667,502,000
Available for borrowing		$ 582,333,000		$ 582,333,000		567,167,000
Dividends declared and paid per common share (in dollars per share)		$ 0.57	$ 0.57	$ 1.14	$ 1.14
Bank Borrowings
Real Estate Investments
Outstanding Balance		$ 85,500,000		$ 85,500,000		96,500,000
Available for borrowing		514,500,000		514,500,000		503,500,000
Bank Borrowings | Subsequent Event
Real Estate Investments
Payments on debt	$ 14,500,000
Outstanding Balance	100,000,000
Available for borrowing	500,000,000
Senior Unsecured Notes
Real Estate Investments
Outstanding Balance		566,940,000		566,940,000		571,002,000
Available for borrowing		$ 67,833,000		$ 67,833,000		$ 63,667,000
Senior Unsecured Notes | Subsequent Event
Real Estate Investments
Payments on debt	14,000,000
Outstanding Balance	552,940,000
Available for borrowing	$ 77,833,000
Common Stock | Dividend Payable, July 31, 2018 | Subsequent Event
Real Estate Investments
Dividends declared and paid per common share (in dollars per share)	$ 0.19
Common Stock | Dividend Payable, August 31, 2018 | Subsequent Event
Real Estate Investments
Dividends declared and paid per common share (in dollars per share)	0.19
Common Stock | Dividend Payable, September 28, 2018 | Subsequent Event
Real Estate Investments
Dividends declared and paid per common share (in dollars per share)	$ 0.19