Common Stock (Details Textual) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended				12 Months Ended
	Jun. 30, 2017	Apr. 30, 2017	Oct. 30, 2016	May 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Proceeds From Issuance Of Common Stock	$ 108,000		$ 95,000	$ 109,600	$ 222,695	$ 228,011	$ 92,260
ATM Program [Member]
Class of Stock [Line Items]
Proceeds From Issuance Of Common Stock					116,500
Gross Amount Remaining To Be Issued					$ 83,500
Common Stock Average Price					$ 49.17
Common Stock [Member]
Class of Stock [Line Items]
Stock Issued During Period, Shares, New Issues	2,415,000		2,087,250	2,875,000
Common Stock [Member] | ATM Program [Member]
Class of Stock [Line Items]
Value Of Equity Instruments To Be Issued		$ 200,000
Stock Issued During Period, Shares, New Issues					2,368,359