Reconciliation of Segment Information, Total Assets (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets:
Total Assets	$ 9,183,981,000	$ 10,929,902,000	$ 11,654,646,000
Items excluded from segment assets:
Income Taxes Receivable, Current	116,851,000	13,887,000
Total Property, Plant and Equipment-Net	8,140,439,000	9,669,377,000
Deferred Tax Assets	853,542,000	1,027,604,000
Unamortized Debt Issuance Expense	27,699,000	33,017,000
Total Reportable Business Segments
Assets:
Total Assets	8,217,774,000	8,968,585,000
Gas Segment [Member]
Assets:
Total Assets	6,235,568,000	6,892,284,000	7,364,185,000
Items excluded from segment assets:
Total Property, Plant and Equipment-Net	5,744,930,000	6,179,353,000
Corporate and Other [Member]
Assets:
Total Assets	797,830,000	859,675,000
Other Segments [Member]
Assets:
Total Assets	966,124,000	939,497,000	1,186,874,000
Items excluded from segment assets:
Total Property, Plant and Equipment-Net	657,461,000	696,088,000
Excludes Gas Segment [Member]
Items excluded from segment assets:
Cash and Other Investments	47,153,000	65,935,000
Intersegment Eliminations [Member]
Assets:
Total Assets	83,000	1,021,820,000	$ 1,009,546,000
Items excluded from segment assets:
Income Taxes Receivable, Current	116,851,000	13,887,000
Deferred Tax Assets	4,290,000	0
Disposal Group, Including Discontinued Operation, Assets	83,000	1,021,820,000
UNITED STATES
Segment Reporting, Asset Reconciling Item [Line Items]
Disclosure of Long-Live Assets by Geographic Region	8,129,415,000	8,721,682,000
CANADA
Segment Reporting, Asset Reconciling Item [Line Items]
Disclosure of Long-Live Assets by Geographic Region	11,024,000	11,024,000
Discontinued Operations [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Disclosure of Long-Live Assets by Geographic Region	$ 0	$ 936,671,000