Stockholders' Equity of the Company (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Common Stock of the Company [Abstract]
At Market Stock Offering Program Aggregate Value Of Common Stock	$ 200,000,000
Proceeds from Issuance of Common Stock	23,395,000	382,063,000
Common stock, shares issued	75,349,705		74,926,981
At Market Stock Offering Program Remained Available For Issuance	126,100,000
Issuance of Equity under ATM Program [Member]
Common Stock of the Company [Abstract]
Issuance of common stock, shares	453,679
Average price per common shares sold	$ 52.65
At Market Stock Offering Program Gross Proceeds	23,900,000
Proceeds from Issuance of Common Stock	$ 23,400,000
Common stock, shares issued	1,596,102
Common Stock [Member]
Common Stock of the Company [Abstract]
Issuance of common stock, shares	453,679	9,487,500