Issuance of Partnership Equity (Textuals) (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Subsidiary Sale Of Stock [Line Items]
Proceeds from Issuance of Common Limited Partners Units	$ 185,528,000	$ 0
Public Placement [Member]
Subsidiary Sale Of Stock [Line Items]
Proceeds from Debt, Net of Issuance Costs	125,400,000
Limited Partners Unit Price Per Unit	$ 15.15
Partners Capital Account Units Sold In Public Offering	8,625,000
Private Placement [Member]
Subsidiary Sale Of Stock [Line Items]
Limited Partners Unit Price Per Unit	$ 14.55
Partners Capital Account Units Sold In Private Placement	2,700,000
Proceeds from Issuance of Private Placement	39,200,000
Equity Distribution [Member]
Subsidiary Sale Of Stock [Line Items]
Aggregate Amount Of Equity Securities Allowed Under Equity Distribution Agreement	75,000,000
Partners' Capital Account, Units, Sale of Units	1,200,000
Sales Agent Commissions	300,000
Proceeds from Issuance of Common Limited Partners Units	$ 20,900,000