Transactions With Affiliates (Narrative) (Details) - USD ($)
$ in Millions |
|
3 Months Ended |
9 Months Ended |
|
|
|
Jun. 24, 2016 |
Sep. 30, 2016 |
Sep. 30, 2015 |
Sep. 30, 2016 |
Sep. 30, 2015 |
Jul. 01, 2016 |
Dec. 31, 2015 |
Dec. 05, 2014 |
| Related Party Transaction [Line Items] |
|
|
|
|
|
|
|
|
| Merger Related Utilities Operating Expense Maintenance and Operations Related Parties |
|
$ 78.3
|
|
|
|
|
|
|
| Interest Expense, Long-term Debt |
|
7.6
|
$ 7.7
|
$ 22.8
|
$ 27.4
|
|
|
|
| Allowance for Funds Used During Construction, Capitalized Interest |
|
1.8
|
1.7
|
3.7
|
4.1
|
|
|
|
| Interest Expense, Short-term Borrowings |
|
2.6
|
$ 0.5
|
3.5
|
0.9
|
|
|
|
| Accounts receivable |
|
161.2
|
|
161.2
|
|
|
$ 149.4
|
|
| Line of Credit Facility, Maximum Borrowing Capacity |
|
2,600.0
|
|
2,600.0
|
|
|
|
|
| Short-term borrowings-affiliated |
|
1,121.5
|
|
1,121.5
|
|
|
42.1
|
|
| Distribution to Parent |
|
|
|
373.0
|
$ 569.9
|
|
|
|
| Transportation and Storage Services |
|
|
|
|
|
|
|
|
| Related Party Transaction [Line Items] |
|
|
|
|
|
|
|
|
| Accounts receivable |
|
$ 1.7
|
|
$ 1.7
|
|
|
8.9
|
|
| Money Pool |
|
|
|
|
|
|
|
|
| Related Party Transaction [Line Items] |
|
|
|
|
|
|
|
|
| Short-term Debt, Weighted Average Interest Rate |
|
1.68%
|
1.21%
|
1.68%
|
1.21%
|
|
|
|
| Accounts receivable |
|
$ 159.5
|
|
$ 159.5
|
|
|
$ 140.5
|
|
| Line of Credit Facility, Maximum Borrowing Capacity |
|
750.0
|
|
750.0
|
|
|
|
|
| Short-term borrowings-affiliated |
|
1,116.5
|
|
1,116.5
|
|
|
|
|
| Revolving Credit Facility |
|
|
|
|
|
|
|
|
| Related Party Transaction [Line Items] |
|
|
|
|
|
|
|
|
| Line of Credit Facility, Maximum Borrowing Capacity |
|
500.0
|
|
500.0
|
|
|
|
$ 1,500.0
|
| Notes Due 2020 |
|
|
|
|
|
|
|
|
| Related Party Transaction [Line Items] |
|
|
|
|
|
|
|
|
| Line of Credit Facility, Maximum Borrowing Capacity |
|
$ 2,300.0
|
|
$ 2,300.0
|
|
|
|
|
| Debt Instrument, Interest Rate, Stated Percentage |
|
4.70%
|
|
4.70%
|
|
|
|
|
| Debt Instrument, Maturity Date |
|
|
|
Dec. 31, 2020
|
|
|
|
|
| Columbia OpCo |
|
|
|
|
|
|
|
|
| Related Party Transaction [Line Items] |
|
|
|
|
|
|
|
|
| Reimbursement of Preformation Capital |
|
|
|
|
$ 500.0
|
|
|
|
| London Interbank Offered Rate (LIBOR) |
|
|
|
|
|
|
|
|
| Related Party Transaction [Line Items] |
|
|
|
|
|
|
|
|
| Debt Instrument, Basis Spread on Variable Rate |
1.08%
|
|
|
|
|
|
|
|
| Revolving Credit Facility |
|
|
|
|
|
|
|
|
| Related Party Transaction [Line Items] |
|
|
|
|
|
|
|
|
| Short-term Debt, Weighted Average Interest Rate |
|
1.79%
|
|
1.79%
|
|
|
1.28%
|
|
| Short-term borrowings-affiliated |
|
$ 5.0
|
|
$ 5.0
|
|
|
|
|
| TransCanada PipeLine USA Ltd. | Revolving Credit Facility |
|
|
|
|
|
|
|
|
| Related Party Transaction [Line Items] |
|
|
|
|
|
|
|
|
| Line of Credit Facility, Maximum Borrowing Capacity |
|
|
|
|
|
$ 2,000.0
|
|
|
| Columbia Energy Group | Revolving Credit Facility |
|
|
|
|
|
|
|
|
| Related Party Transaction [Line Items] |
|
|
|
|
|
|
|
|
| Line of Credit Facility, Maximum Borrowing Capacity |
$ 50.0
|
|
|
|
|
|
|
|