Accumulated Other Comprehensive Loss (Narrative) (Details)
$ in Millions |
|
12 Months Ended |
|
Aug. 31, 2010
USD ($)
|
Dec. 31, 2015
USD ($)
|
Dec. 31, 2014
USD ($)
|
Dec. 31, 2008
USD ($)
|
| Accumulated Other Comprehensive Income (Loss) [Line Items] |
|
|
|
|
| Number Of Notes Tranches Through Private Placement Issuance |
|
2
|
|
|
| Millennium Pipeline |
|
|
|
|
| Accumulated Other Comprehensive Income (Loss) [Line Items] |
|
|
|
|
| Number of interest rate swap agreements |
|
|
|
3
|
| Notional amount of interest rate swap agreements |
|
|
|
$ 420.0
|
| Number of counterparties entered into interest rate swap agreement |
|
|
|
7
|
| Refinancing of long-term debt |
$ 725.0
|
|
|
|
| Debt Instrument, Maturity Date |
|
Jun. 01, 2025
|
|
|
| Other Comprehensive Income Unrecognized Gain Loss On Derivatives Arising During Period Net Of Tax |
|
$ 25.0
|
$ 16.6
|
|
| Unrealized loss, amortization period |
|
15 years
|
|
|
| Notes Due 2027 [Member] | Millennium Pipeline |
|
|
|
|
| Accumulated Other Comprehensive Income (Loss) [Line Items] |
|
|
|
|
| Refinancing of long-term debt |
$ 375.0
|
|
|
|
| Debt Instrument, Interest Rate, Stated Percentage |
5.33%
|
|
|
|
| Debt Instrument, Maturity Date |
Jun. 30, 2027
|
|
|
|
| Notes Due 2032 | Millennium Pipeline |
|
|
|
|
| Accumulated Other Comprehensive Income (Loss) [Line Items] |
|
|
|
|
| Refinancing of long-term debt |
$ 350.0
|
|
|
|
| Debt Instrument, Interest Rate, Stated Percentage |
6.00%
|
|
|
|
| Debt Instrument, Maturity Date |
Jun. 30, 2032
|
|
|
|