Equity Method Investments (Investment Summary) (Details) - USD ($)		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Equity method investments		$ 274,009,000				$ 285,281,000				$ 274,009,000	$ 285,281,000
Earnings from equity method investments		16,179,000	$ 14,528,000	$ 14,351,000	$ 12,614,000	15,120,000	$ 17,367,000	$ 17,753,000	$ 17,091,000	57,672,000	67,331,000	$ 73,757,000
Loss on issuance of common units by equity method investee										0	0	(41,000)
Proceeds from Equity Method Investment, Dividends or Distributions, Return of and Return on Capital										76,725,000	95,522,000	104,168,000
White Cliffs Pipeline, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments		255,043,000				266,362,000				255,043,000	266,362,000
Earnings from equity method investments										57,625,000	59,851,000	69,007,000
Proceeds from Equity Method Investment, Dividends or Distributions, Return of and Return on Capital										76,725,000	77,511,000	88,839,000
Ngl Energy Partners Lp [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments		$ 18,966,000				$ 18,919,000				18,966,000	18,919,000
Earnings from equity method investments	[1]									47,000	(14,000)	2,188,000
Loss on issuance of common units by equity method investee												(41,000.0)
Proceeds from Equity Method Investment, Dividends or Distributions, Return of and Return on Capital										0	0	4,873,000
Glass Mountain Pipeline LLC [Member]
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments										0	7,494,000	2,562,000
Proceeds from Equity Method Investment, Dividends or Distributions, Return of and Return on Capital										$ 0	$ 18,011,000	$ 10,456,000

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