Stockholders' Equity of the Company (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2015	Sep. 30, 2014	Jul. 01, 2015	Dec. 31, 2014
Common Stock of the Company [Abstract]
Net proceeds from issuance of common stock (Note 8)		$ 387,509	$ 22,136
Common stock, shares issued				3,773,766
Shares Issued, Price Per Share				$ 66.19
At Market Stock Aggregate Gross Sales Price of Common Stock				$ 249,800
Proceeds from Issuance of Common stock, net	$ 249,600
Common Stock [Member]
Common Stock of the Company [Abstract]
Common stock, shares issued	88,405,632	88,405,632			86,259,684
Issuance of Equity - at the market offering [Member] | Common Stock [Member]
Common Stock of the Company [Abstract]
At Market Stock Aggregate Gross Sales Price of Common Stock	$ 150,100	$ 150,100
December 2014 At-The-Market Program [Member] | Common Stock [Member]
Common Stock of the Company [Abstract]
Common stock, shares issued	2,007,767	2,007,767
Issuance of Equity - at the market offering [Member] | Common Stock [Member]
Class of Stock [Line Items]
At Market Stock Offering Program Aggregate Value Of Common Stock		$ 300,000
Common Stock of the Company [Abstract]
Issuance of common stock, shares (Note 8)		1,866,267
Average price per common shares sold		$ 75.06
Net proceeds from issuance of common stock (Note 8)		$ 140,100
At the Market Stock Offering Program Remained Available For Issuance		149,900
Proceeds from Issuance of Common stock, net		$ 138,200