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Partners' Capital - Additional Information (Detail) (USD $)
In Millions, except Share data, unless otherwise specified
1 Months Ended 3 Months Ended 12 Months Ended 0 Months Ended 1 Months Ended 12 Months Ended 0 Months Ended 12 Months Ended 12 Months Ended
Dec. 21, 2011
Sep. 30, 2012
Jun. 30, 2012
Mar. 31, 2012
Sep. 30, 2012
Classes_of_Unitholders
Nov. 14, 2012
Subsequent Event
Nov. 07, 2012
Subsequent Event
Dec. 21, 2011
Underwriters Option To Purchase Units
Sep. 30, 2012
Restricted units
Nov. 14, 2012
Dividend Paid [Member]
Subsequent Event
Sep. 30, 2012
Inergy Midstream
Sep. 30, 2012
Inergy Midstream
Long Term Incentive Plan Member
Sep. 30, 2012
Inergy Midstream
Restricted units
Distribution Made to Member or Limited Partner [Line Items]                          
Unit-based compensation charges                         $ 0.7
Number of Classes of Unitholders         3                
Maximum period for distribution of available cash after the end of each quarter         45 days                
Percentage of cash distributions entitled to receive from operating surplus         50.00%                
Distribution declared per limited partner unit   $ 0.38 [1] $ 0.37 [1] $ 0.04 [1] $ 0.37         $ 0.385      
Number of common stock units issued due to underwriter exercise of over allotment provision 18,400,000             2,400,000          
Distribution to limited partner, record date   Aug. 07, 2012 May 08, 2012 Feb. 07, 2012     Nov. 07, 2012            
Distribution to limited partner, distribution date   Aug. 14, 2012 May 15, 2012 Feb. 14, 2012   Nov. 14, 2012              
Total cash distribution   29.3 27.6 3.0 59.9                
Annual cash distributions paid per limited partner unit         $ 1.48                
Incentive common units available                       7,432,500  
Vested Percentage, Year Five                         100.00%
Vested Percentage, After Year Three                         25.00%
Vested Percentage, After Year Four                         25.00%
Vesting Percentage, After Year Five                         50.00%
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]                          
Non-vested at October 1, 2011                     $ 0.00    
Granted during the period ended September 30, 2012                     $ 21.62    
Vested during the period ended September 30, 2012                     $ 0.00    
Forfeited during the period ended September 30, 2012                     $ 0.00    
Non-vested at September 30, 2012                     $ 21.62    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]                          
Non-vested at October 1, 2011                         0
Granted during the period endd September 30, 2012                         383,223
Vested in period ended September 30, 2012                         0
Forfeited in period ended September 30, 2012                         0
Non-vested at September 30, 2012                 383,223        
Unrecognized compensation cost                 $ 7.6        
Unrecognized compensation cost, period for recognition                 5 years        
[1] The $0.04 cash distribution per limited partner unit corresponds to an initial quarterly cash distribution of $0.37 per quarter ($1.48 annually) and represents the prorated distribution for the period of time from December 21, 2011, the closing of the Company's initial public offering, through December 31, 2011, the end of the first fiscal quarter.