Common Stock (Details Textual) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		3 Months Ended	9 Months Ended
	Jun. 30, 2017	Apr. 30, 2017	Sep. 30, 2017	Sep. 30, 2017	Sep. 30, 2016
Class of Stock [Line Items]
Proceeds From Issuance Of Common Stock	$ 108,200			$ 136,733	$ 124,099
ATM Program [Member]
Class of Stock [Line Items]
Proceeds From Issuance Of Common Stock			$ 29,200	29,300
Gross Amount Remaining To Be Issued				$ 170,700
Common Stock Average Price			$ 49.58	$ 49.57
Common Stock [Member]
Class of Stock [Line Items]
Stock Issued During Period, Shares, New Issues	2,415,000
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			589,093	591,593