Equity Method Investments (Investment Summary) (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Schedule of Equity Method Investments [Line Items]
Gain on issuance of common units by equity method investee		$ 352	$ 136	$ 5,897	$ 0	$ 2,121	$ 18,772	$ 0	$ 8,127	$ 6,385	$ 29,020	$ 26,873
Equity method investments		551,078				577,920				551,078	577,920
Earnings from equity method investments		20,687	$ 16,237	$ 23,903	$ 20,559	15,827	$ 14,223	$ 19,187	$ 14,962	81,386	64,199	52,477
Proceeds from Equity Method Investment, Dividends or Distributions, Return of and Return on Capital										119,542	96,995	75,897
White Cliffs Pipeline, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments		297,109				269,635				297,109	269,635
Earnings from equity method investments										70,238	57,378	45,459
Proceeds from Equity Method Investment, Dividends or Distributions, Return of and Return on Capital										86,845	66,768	57,576
Ngl Energy Partners Lp [Member]
Schedule of Equity Method Investments [Line Items]
Gain on issuance of common units by equity method investee										6,400	29,000	26,900
Equity method investments		112,787				162,246				112,787	162,246
Earnings from equity method investments	[1]									5,031	2,343	7,123
Proceeds from Equity Method Investment, Dividends or Distributions, Return of and Return on Capital										19,074	23,404	18,321
Glass Mountain Pipeline LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments		$ 141,182				$ 146,039				141,182	146,039
Earnings from equity method investments										6,117	4,478	(105)
Proceeds from Equity Method Investment, Dividends or Distributions, Return of and Return on Capital										$ 13,623	$ 6,823	$ 0

[1] (1) Excluding gains on issuance of common units of $6.4 million, $29.0 million and $26.9 million for the years ended December 31, 2015, 2014 and 2013, respectively.