Partners' Capital and Distributions (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands					12 Months Ended
	Jul. 22, 2015	Apr. 10, 2015	Sep. 26, 2014	Sep. 16, 2013	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014
Partners Capital And Distributions [Line Items]
Partners' Capital Account, Units, Sold in Public Offering	10,350,000	4,600,000	4,600,000	5,750,000
Partners' Capital Account, Public Sale of Units Net of Offering Costs	$ 437,164	$ 198,168	$ 225,725	$ 263,574
Common units, outstanding					109,979,218				95,029,218
Per Unit Amount					$ 0.6400	$ 0.6250	$ 0.6100	$ 0.5950	$ 0.5800	$ 0.5650	$ 0.5500	$ 0.5350
Common units, issued	10,350,000	4,600,000	4,600,000	5,750,000	109,979,218				95,029,218
Cash or stock available for distributions, percent usually distributed					100.00%
Days to distribute					45 days
Price per share of common units issued	$ 43.77	$ 44.42	$ 50.71	$ 47.51
Partners' Capital Account, Public Sale of Units	$ 453,020	$ 204,332	$ 233,266	$ 273,183
Partners' Capital Account, Costs In Connection With Common Units Issued To The Public	$ (15,856)	$ (6,164)	(7,541)	(9,609)
Limited Partners' Capital Accountant, Option To Purchase	1,350,000	600,000
Waiver Units Class 2 [Member]
Partners Capital And Distributions [Line Items]
Waiver units authorized					1,738,000
Cash or stock available for distributions, percent usually distributed					100.00%
Days to distribute					45 days
Waiver Units Class 3 [Member]
Partners Capital And Distributions [Line Items]
Waiver units authorized					1,738,000
Cash or stock available for distributions, percent usually distributed					100.00%
Days to distribute					45 days
Waiver Units Class4 [Member]
Partners Capital And Distributions [Line Items]
Waiver units authorized					1,738,000
Cash or stock available for distributions, percent usually distributed					100.00%
Days to distribute					45 days
Class A [Member]
Partners Capital And Distributions [Line Items]
Partners' Capital Account, Public Sale of Units Net of Offering Costs			$ 225,700	$ 263,600
Common units, outstanding					109,939,221
Class B [Member]
Partners Capital And Distributions [Line Items]
Common units, outstanding					39,997
Class Two [Member]
Partners Capital And Distributions [Line Items]
Conversion feature, minimum distribution coverage ratio					1.1
Class Three [Member]
Partners Capital And Distributions [Line Items]
Conversion feature, minimum distribution coverage ratio					1.1
Class Four [Member]
Partners Capital And Distributions [Line Items]
Conversion feature, minimum distribution coverage ratio					1.1