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Description of Business and Summary of Significant Accounting Policies Equity Company Investments (Tables)
 12 Months Ended
Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]  
Equity Method Investments [Table Text Block]
 
 
Balance at December 31, 2016
 
 
Underlying Equity
 
Recorded Investment Balance
 
Difference
 
 
(in thousands)
Equity Method Investment
 
 
 
 
 
 
SLC Pipeline LLC
 
$
57,273

 
$
24,417

 
$
32,856

Frontier Aspen LLC
 
11,630

 
53,160

 
(41,530
)
Osage Pipe Line Company, LLC
 
10,730

 
43,375

 
(32,645
)
Cheyenne Pipeline LLC
 
29,658

 
44,657

 
(14,999
)
Total
 
$
109,291

 
$
165,609

 
$
(56,318
)