Certain Provisions of the Partnership Agreement (Issuance of Common Units) (Details) (USD $)	0 Months Ended		6 Months Ended
	Jun. 14, 2013	Jan. 14, 2013	Jun. 30, 2013	Jun. 30, 2012
Distribution Made To Common And Preferred Units Holders [Line Items]
Common Stock Issuance Date	Jun. 14, 2013	Jan. 14, 2013
Partners' Capital Account, Units, Sold in Public Offering	8,280,000
Limited Partners Unit Price Per Unit	$ 20.33
Issuance of common units by the Partnership	$ 162,000,000		$ 362,778,000	$ 158,014,000
Minority Interest Decrease From Stock Issuance			24,200,000
Deferred Tax Liabilities Investments Decrease			3,200,000
Greenshoe Units Sold [Member]
Distribution Made To Common And Preferred Units Holders [Line Items]
Partners' Capital Account, Units, Sold in Public Offering	1,080,000
Public Placement [Member]
Distribution Made To Common And Preferred Units Holders [Line Items]
Partners' Capital Account, Units, Sold in Public Offering		8,625,000
Limited Partners Unit Price Per Unit		$ 15.15
Issuance of common units by the Partnership		125,400,000
Private Placement [Member]
Distribution Made To Common And Preferred Units Holders [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]
Distribution Made To Common And Preferred Units Holders [Line Items]
Issuance of common units by the Partnership			36,400,000
Sales Agent Commissions			600,000
Aggregate Amount Of Equity Securities Allowed Under Equity Distribution Agreement			$ 75,000,000.0
Partners' Capital Account, Units, Sale of Units			1,947,576