Common Stock (Details Textual) - USD ($) $ in Thousands	1 Months Ended			9 Months Ended
	Sep. 30, 2018	May 31, 2018	Jun. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Mar. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Proceeds from Issuance of Common Stock			$ 108,000	$ 159,604	$ 136,733
Common Stock, Shares Authorized	45,000,000			45,000,000			45,000,000
Forward Contracts Two [Member]
Class of Stock [Line Items]
Common Stock, Shares Authorized	3,500,000			3,500,000
Common Stock [Member]
Class of Stock [Line Items]
Proceeds from Issuance of Common Stock	$ 160,200			$ 190,000
Stock Issued During Period, Shares, New Issues			2,415,000	3,450,000
Common Stock [Member] | Forward Contracts [Member]
Class of Stock [Line Items]
Stock Issued During Period, Shares, New Issues				3,450,000
Common Stock, Shares Authorized						3,450,000
ATM Program [Member]
Class of Stock [Line Items]
Gross Amount Remaining To Be Issued				$ 250,000
ATM Program [Member] | Common Stock [Member]
Class of Stock [Line Items]
Value Of Equity Instruments To Be Issued		$ 250,000
Over-Allotment Option [Member] | Common Stock [Member] | Forward Contracts [Member]
Class of Stock [Line Items]
Common Stock, Shares Authorized						450,000