Initial Public Offering (Narrative) (Details) - USD ($)
$ / shares in Units, $ in Millions |
1 Months Ended |
8 Months Ended |
9 Months Ended |
Feb. 11, 2015 |
Sep. 30, 2015 |
Sep. 30, 2016 |
Sep. 30, 2015 |
| Initial Public Offering [Line Items] |
|
|
|
|
| Contribution of capital from parent |
$ 1,217.3
|
|
|
|
| Limited Liability Company (LLC) or Limited Partnership (LP), Members or Limited Partners, Ownership Interest Additional Ownership Interest |
|
|
46.50%
|
|
| Net proceeds from IPO |
|
$ 1,168.4
|
|
|
| Underwriting discounts, structuring fees, and offering expenses |
|
$ 69.8
|
|
|
| Limited Liability Company (LLC) or Limited Partnership (LP), Members or Limited Partners, Ownership Interest |
|
|
15.70%
|
|
| Common Units |
|
|
|
|
| Initial Public Offering [Line Items] |
|
|
|
|
| Shares Issued, Price Per Share |
$ 23.00
|
|
|
|
| Partners' Capital Account, Units, Sold in Public Offering |
|
53,833,107
|
|
|
| Over-Allotment Option [Member] | Common Units |
|
|
|
|
| Initial Public Offering [Line Items] |
|
|
|
|
| Partners' Capital Account, Units, Sold in Public Offering |
|
7,021,709
|
|
|
| Columbia Energy Group |
|
|
|
|
| Initial Public Offering [Line Items] |
|
|
|
|
| Contribution of capital from parent |
$ 1,217.3
|
|
|
|
| Limited Liability Company (LLC) or Limited Partnership (LP), Members or Limited Partners, Ownership Interest |
|
7.30%
|
|
|
| Columbia Energy Group | Subordinated Units |
|
|
|
|
| Initial Public Offering [Line Items] |
|
|
|
|
| Partners' Capital Account, Units, Sold in Public Offering |
|
46,811,398
|
|
|
| Columbia OpCo |
|
|
|
|
| Initial Public Offering [Line Items] |
|
|
|
|
| Limited Liability Company (LLC) or Limited Partnership (LP), Members or Limited Partners, Ownership Interest Additional Ownership Interest |
|
8.40%
|
|
|
| Limited Liability Company (LLC) or Limited Partnership (LP), Members or Limited Partners, Ownership Interest |
|
15.70%
|
|
|
| Reimbursement of Preformation Capital |
|
|
|
$ 500.0
|
| Limited Partner | Common Units |
|
|
|
|
| Initial Public Offering [Line Items] |
|
|
|
|
| Contribution of capital from parent |
0.0
|
|
|
|
| Net proceeds from IPO |
|
$ 1,168.4
|
|
|
| Limited Partner | Subordinated Units |
|
|
|
|
| Initial Public Offering [Line Items] |
|
|
|
|
| Contribution of capital from parent |
$ 0.0
|
|
|
|
| Net proceeds from IPO |
|
$ 0.0
|
|
|
| Limited Partner | Columbia Energy Group |
|
|
|
|
| Initial Public Offering [Line Items] |
|
|
|
|
| Limited Liability Company (LLC) or Limited Partnership (LP), Members or Limited Partners, Ownership Interest |
|
84.30%
|
|
|