Condensed Consolidated Balance Sheets (Unaudited) (USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010	Sep. 30, 2010
Current assets
Cash and cash equivalents	$ 41.6	$ 31.5	$ 75.1
Short-term investments	52.8	70.6	66.8
Receivables, less allowances of $27.1, $27.6 and $37.7, respectively	231.5	472.4	235.7
Gas in storage	240.2	163.9	227.8
Derivative instruments	49.6	49.1	70.4
Margin accounts - derivative instruments	27.5	50.9	58.8
Other	144.8	115.2	148.8
Total current assets	788.0	953.6	883.4
Property, plant and equipment, at cost [Abstract]
Property, plant and equipment, at cost	5,278.3	5,127.7	5,060.6
Less accumulated depreciation	2,172.6	2,104.9	2,081.1
Total property, plant and equipment, net	3,105.7	3,022.8	2,979.5
Long-term investments	134.6	134.2	133.9
Other assets	381.9	385.9	373.6
Total assets	4,410.2	4,496.5	4,370.4
Current liabilities
Long-term debt due within one year	0	0	75.0
Short-term debt	312.6	425.0	373.3
Accounts payable	305.8	335.5	285.8
Customer credit balances and deposits	109.3	110.6	131.2
Derivative instruments	80.3	82.9	115.9
Other	116.3	120.3	96.3
Total current liabilities	924.3	1,074.3	1,077.5
Deferred credits and other liabilities
Regulatory asset retirement liability	878.5	843.9	836.0
Deferred income taxes	437.1	423.4	416.2
Health care and other postretirement benefits	232.6	229.7	201.9
Asset retirement obligation	197.3	190.9	188.6
Other	140.2	132.0	148.0
Total deferred credits and other liabilities	1,885.7	1,819.9	1,790.7
Commitments and contingencies
Capitalization
Long-term obligations	498.5	498.4	423.4
Common equity
Common stock	113.9	113.9	113.8
Paid-in capital	67.5	69.1	67.0
Retained earnings	939.9	934.1	915.4
Accumulated other comprehensive loss	(19.6)	(13.2)	(17.4)
Total common equity	1,101.7	1,103.9	1,078.8
Total capitalization	1,600.2	1,602.3	1,502.2
Total liabilities and capitalization	4,410.2	4,496.5	4,370.4
Gas Distribution [Member]
Property, plant and equipment, at cost [Abstract]
Property, plant and equipment, at cost	4,841.7	4,733.6	4,680.3
Shipping [Member]
Property, plant and equipment, at cost [Abstract]
Property, plant and equipment, at cost	323.7	333.6	332.2
Other [Member]
Property, plant and equipment, at cost [Abstract]
Property, plant and equipment, at cost	$ 112.9	$ 60.5	$ 48.1