Equity Method Investments (Investment Summary) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 577,920				$ 565,124				$ 577,920		$ 565,124
Earnings from equity method investments	15,827	14,223	19,187	14,962	12,788	7,483	14,861	17,345	64,199		52,477		36,036
Proceeds from Equity Method Investment, Dividends or Distributions, Return of and Return on Capital									96,995		75,897		53,731
White Cliffs Pipeline, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	269,635				224,095				269,635		224,095
Earnings from equity method investments									57,378		45,459		36,439
Proceeds from Equity Method Investment, Dividends or Distributions, Return of and Return on Capital									66,768		57,576		44,514
Ngl Energy Partners Lp [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	162,246				208,848				162,246		208,848
Earnings from equity method investments									2,343	[1]	7,123	[1]	(403)
Proceeds from Equity Method Investment, Dividends or Distributions, Return of and Return on Capital									23,404		18,321		9,217
Glass Mountain Pipeline LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	146,039				132,181				146,039		132,181
Earnings from equity method investments									4,478		(105)		0
Proceeds from Equity Method Investment, Dividends or Distributions, Return of and Return on Capital									$ 6,823		$ 0		$ 0

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