Equity Method Investments (Investment Summary) (Details) - USD ($)		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Equity Method Investments [Line Items]
Gain (loss) on issuance of common units by equity method investee						$ 0	$ 0	$ 0	$ (41,000)	$ 0	$ (41,000)	$ 6,385,000
Equity method investments		$ 285,281,000				434,289,000				285,281,000	434,289,000
Earnings from equity method investments		15,120,000	$ 17,367,000	$ 17,753,000	$ 17,091,000	17,763,000	$ 15,845,000	$ 17,078,000	$ 23,071,000	67,331,000	73,757,000	81,386,000
Proceeds from Equity Method Investment, Dividends or Distributions, Return of and Return on Capital										95,522,000	104,168,000	119,542,000
White Cliffs Pipeline, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments		266,362,000				281,734,000				266,362,000	281,734,000
Earnings from equity method investments										59,851,000	69,007,000	70,238,000
Proceeds from Equity Method Investment, Dividends or Distributions, Return of and Return on Capital										77,511,000	88,839,000	86,845,000
Ngl Energy Partners Lp [Member]
Schedule of Equity Method Investments [Line Items]
Gain (loss) on issuance of common units by equity method investee											(41,000)	6,400,000
Equity method investments		18,919,000				18,933,000				18,919,000	18,933,000
Earnings from equity method investments	[1]									(14,000)	2,188,000	5,031,000
Proceeds from Equity Method Investment, Dividends or Distributions, Return of and Return on Capital										0	4,873,000	19,074,000
Glass Mountain Pipeline LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments		$ 0				$ 133,622,000				0	133,622,000
Earnings from equity method investments										7,494,000	2,562,000	6,117,000
Proceeds from Equity Method Investment, Dividends or Distributions, Return of and Return on Capital										$ 18,011,000	$ 10,456,000	$ 13,623,000

[1] (1) Excluding a loss on issuance of common units of $41.0 thousand for the year ended December 31, 2016, and a gain on the issuance of common units of $6.4 million for the year ended December 31, 2015.