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Debt and Interest Expense - Additional Information (Details) - USD ($)
3 Months Ended
Feb. 15, 2019
Mar. 31, 2019
Mar. 31, 2018
Feb. 28, 2019
Dec. 31, 2018
Dec. 19, 2018
Feb. 16, 2018
Jun. 30, 2017
Debt Instrument [Line Items]                
APCWH Note Payable, maximum borrowing [1]   $ 0     $ 427,493,000 [2]      
Gain (loss) on interest-rate swaps   (35,638,000) $ 0 [2]          
WES [Member] | Revolving Credit Facility [Member] | WGP RCF [Member]                
Debt Instrument [Line Items]                
Facility, maximum borrowing capacity             $ 35,000,000.0  
Repayments of facility borrowings   28,000,000            
WES Operating [Member]                
Debt Instrument [Line Items]                
APCWH Note Payable, maximum borrowing [3]   0     427,493,000 [2]      
Gain (loss) on interest-rate swaps   (35,638,000) $ 0 [2]          
WES Operating [Member] | Not Designated as Hedging Instrument [Member]                
Debt Instrument [Line Items]                
Interest-rate swap, notional amount   375,000,000     750,000,000      
WES Operating [Member] | Interest-Rate Swap [Member]                
Debt Instrument [Line Items]                
Gain (loss) on interest-rate swaps   (35,638,000)            
WES Operating [Member] | Interest-Rate Swap [Member] | Not Designated as Hedging Instrument [Member]                
Debt Instrument [Line Items]                
Interest-rate swap, collateral posted if credit risk related provision triggered   18,600,000            
Interest-rate swap, aggregate fair value of swaps with credit risk related features for which a net liability position existed   35,100,000     5,700,000      
WES Operating [Member] | Interest-Rate Swap [Member] | Not Designated as Hedging Instrument [Member] | Accrued Liabilities [Member]                
Debt Instrument [Line Items]                
Interest-rate swap, liability fair value   $ 43,600,000     $ 8,000,000      
WES Operating [Member] | APCWH Note Payable [Member]                
Debt Instrument [Line Items]                
APCWH Note Payable, interest rate     2.54%          
WES Operating [Member] | APCWH Note Payable [Member] | Maximum [Member]                
Debt Instrument [Line Items]                
APCWH Note Payable, maximum borrowing               $ 500,000,000.0
WES Operating [Member] | Revolving Credit Facility [Member] | RCF [Member]                
Debt Instrument [Line Items]                
Facility, maximum borrowing capacity       $ 2,000,000,000.0     $ 1,500,000,000.0  
Facility, expiration date Feb. 15, 2024              
Facility, accordion feature amount       500,000,000.0        
Facility, expandable maximum borrowing capacity       $ 2,500,000,000.0        
Facility, fee rate   0.20% 0.20%          
Facility, outstanding borrowings   $ 640,000,000            
Facility, available borrowing capacity   $ 1,400,000,000            
Facility, interest rate at period end   3.79% 3.18%          
Facility, outstanding letters of credit   $ 4,600,000            
WES Operating [Member] | Revolving Credit Facility [Member] | RCF [Member] | Minimum [Member]                
Debt Instrument [Line Items]                
Facility, fee rate   0.125%            
WES Operating [Member] | Revolving Credit Facility [Member] | RCF [Member] | Maximum [Member]                
Debt Instrument [Line Items]                
Facility, fee rate   0.25%            
WES Operating [Member] | Revolving Credit Facility [Member] | RCF [Member] | London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member]                
Debt Instrument [Line Items]                
Facility, applicable margin added   1.00%            
WES Operating [Member] | Revolving Credit Facility [Member] | RCF [Member] | London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member]                
Debt Instrument [Line Items]                
Facility, applicable margin added   1.50%            
WES Operating [Member] | Revolving Credit Facility [Member] | RCF [Member] | London Interbank Offered Rate (LIBOR) [Member] | Alternate Base Rate [Member]                
Debt Instrument [Line Items]                
Facility, applicable margin added   1.00%            
WES Operating [Member] | Revolving Credit Facility [Member] | RCF [Member] | Percentage Above Federal Funds Effective Rate [Member] | Alternate Base Rate [Member]                
Debt Instrument [Line Items]                
Facility, applicable margin added   0.50%            
WES Operating [Member] | Revolving Credit Facility [Member] | RCF [Member] | Base Rate [Member] | Minimum [Member]                
Debt Instrument [Line Items]                
Facility, applicable margin added   0.00%            
WES Operating [Member] | Revolving Credit Facility [Member] | RCF [Member] | Base Rate [Member] | Maximum [Member]                
Debt Instrument [Line Items]                
Facility, applicable margin added   0.50%            
WES Operating [Member] | Revolving Credit Facility [Member] | WES 364-Day Facility [Member]                
Debt Instrument [Line Items]                
Facility, maximum borrowing capacity           $ 2,000,000,000.0    
Facility, outstanding borrowings   $ 2,000,000,000.0            
Facility, interest rate at period end   3.87%            
WES Operating [Member] | Revolving Credit Facility [Member] | WES 364-Day Facility [Member] | London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member]                
Debt Instrument [Line Items]                
Facility, applicable margin added   1.00%            
WES Operating [Member] | Revolving Credit Facility [Member] | WES 364-Day Facility [Member] | London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member]                
Debt Instrument [Line Items]                
Facility, applicable margin added   1.625%            
WES Operating [Member] | Revolving Credit Facility [Member] | WES 364-Day Facility [Member] | London Interbank Offered Rate (LIBOR) [Member] | Alternate Base Rate [Member]                
Debt Instrument [Line Items]                
Facility, applicable margin added   1.00%            
WES Operating [Member] | Revolving Credit Facility [Member] | WES 364-Day Facility [Member] | Percentage Above Federal Funds Effective Rate [Member] | Alternate Base Rate [Member]                
Debt Instrument [Line Items]                
Facility, applicable margin added   0.50%            
WES Operating [Member] | Revolving Credit Facility [Member] | WES 364-Day Facility [Member] | Base Rate [Member] | Minimum [Member]                
Debt Instrument [Line Items]                
Facility, applicable margin added   0.00%            
WES Operating [Member] | Revolving Credit Facility [Member] | WES 364-Day Facility [Member] | Base Rate [Member] | Maximum [Member]                
Debt Instrument [Line Items]                
Facility, applicable margin added   0.625%            
[1] See Note 1 and Note 6.
[2] Financial information has been recast to include the financial position and results attributable to AMA. See Note 1 and Note 3.
[3] See Note 1 and Note 6.