Certain Provisions of the Partnership Agreement (Issuance of Common Units) (Details) (USD $)	0 Months Ended		9 Months Ended		3 Months Ended			0 Months Ended			9 Months Ended
	Jun. 14, 2013	Jan. 14, 2013	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 Preferred Stock P I K [Member]	Jun. 30, 2013 Preferred Stock P I K [Member]	Mar. 31, 2013 Preferred Stock P I K [Member]	Jun. 14, 2013 Greenshoe Units Sold [Member]	Jan. 14, 2013 Public Placement [Member]	Jan. 14, 2013 Private Placement [Member]	Sep. 30, 2013 Equity Distribution [Member]
Distribution Made To Common And Preferred Units Holders [Line Items]
Preferred Stock Paid In Kind					394,313	384,731	375,382
Common Stock Issuance Date	Jun. 14, 2013	Jan. 14, 2013
Partners' Capital Account, Units, Sold in Public Offering	8,280,000							1,080,000	8,625,000
Limited Partners Unit Price Per Unit	$ 20.33								$ 15.15	$ 14.55
Issuance of common units by the Partnership	$ 162,000,000		$ 389,186,000	$ 232,791,000					$ 125,400,000		$ 62,900,000
Sales Agent Commissions											900,000
Partners' Capital Account, Units, Sold in Private Placement										2,700,000
Proceeds from Issuance of Private Placement										39,200,000
Aggregate Amount Of Equity Securities Allowed Under Equity Distribution Agreement											75,000,000.0
Partners' Capital Account, Units, Sale of Units											3,370,486
Minority Interest Decrease From Stock Issuance			31,700,000
Deferred Tax Liabilities Investments Decrease			$ 6,000,000
Distribution Made to Limited Partner, Distributions Declared, Per Unit			$ 0.34