Stockholders' Equity of the Company (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Common Stock of the Company [Abstract]
Net proceeds from issuance of common stock (Note 8)	$ 113,097,000	$ 0
Common Stock [Member]
Common Stock of the Company [Abstract]
Common stock, shares issued	88,031,377		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	124,700,000
March 2015 At-The-Market Program [Member]
Common Stock of the Company [Abstract]
Common stock, shares issued	1,648,893
Issuance of Equity under ATM Program [Member] | Common Stock [Member]
Class of Stock [Line Items]
At Market Stock Offering Program Aggregate Value Of Common Stock	300,000,000
Common Stock of the Company [Abstract]
Issuance of common stock, shares (Note 8)	1,507,393
Average price per common shares sold	$ 76.09
Net proceeds from issuance of common stock (Note 8)	114,700,000
At the Market Stock Offering Program Remained Available For Issuance	175,300,000
Proceeds from Issuance of Common stock, net	$ 113,200,000