Stockholders' Equity of the Company (Details) (USD $)	9 Months Ended				0 Months Ended		3 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 Common Stock [Member]	Dec. 31, 2012 Common Stock [Member]	Sep. 18, 2013 Issuance of Equity - equity offering [Domain] Common Stock [Member]	Sep. 30, 2013 Issuance of Equity - at the market ofering [Member]	Sep. 30, 2013 Issuance of Equity - at the market ofering [Member] Common Stock [Member]	Sep. 30, 2013 Issuance of Equity - at the market ofering [Member] Common Stock [Member]
Common Stock of the Company [Abstract]
Issuance of common stock, shares					6,175,000		226,430	1,040,838
Proceeds from Issuance of Common stock, net					$ 295,900,000		$ 11,400,000	$ 54,400,000
At the Market Stock Offering Program Aggregate Value Of Common Stock								200,000,000
Average price per common shares sold							$ 51.12	$ 53.11
Net proceeds from issuance of common stock (Note 7)	349,951,000	662,371,000					11,600,000	55,300,000
Common stock, shares issued			82,113,491	74,926,981		2,183,261
At Market Stock Aggregate Gross Sales Price of Common Stock							105,300,000	105,300,000
At the Market Stock Offering Program Remained Available For Issuance								$ 94,700,000