Accumulated Other Comprehensive Income (Loss) Changes in AOCI, net of tax (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss), net of tax	$ (24,282)
Other Comprehensive (Income) Loss, Reclassification of Stranded Taxes to Retained Earnings	(5,200)
Accumulated other comprehensive income (loss), net of tax	(77,202)	$ (24,282)
Subsidiaries [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss), net of tax	(126,906)
Other Comprehensive (Income) Loss, Reclassification of Stranded Taxes to Retained Earnings	(27,332)
Accumulated other comprehensive income (loss), net of tax	(190,884)	(126,906)
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss), net of tax	(24,282)	(33,712)	$ (27,266)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(48,870)	10,251	(5,528)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	1,180	(821)	(918)
Other Comprehensive (Income) Loss, Reclassification of Stranded Taxes to Retained Earnings	(5,230)
Other Comprehensive Income (Loss), Net of Tax	(52,920)	9,430	(6,446)
Accumulated other comprehensive income (loss), net of tax	(77,202)	(24,282)	(33,712)
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] | Subsidiaries [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss), net of tax	(121,867)	(140,155)	(143,877)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(48,802)	10,200	(5,655)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	11,772	8,088	9,377
Other Comprehensive (Income) Loss, Reclassification of Stranded Taxes to Retained Earnings	(26,233)
Other Comprehensive Income (Loss), Net of Tax	(63,263)	18,288	3,722
Accumulated other comprehensive income (loss), net of tax	(185,130)	(121,867)	(140,155)
Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss), net of tax	(24,282)	(33,712)	(27,266)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(48,870)	10,251	(5,528)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	1,180	(821)	(918)
Other Comprehensive (Income) Loss, Reclassification of Stranded Taxes to Retained Earnings	(5,230)
Other Comprehensive Income (Loss), Net of Tax	(52,920)	9,430	(6,446)
Accumulated other comprehensive income (loss), net of tax	(77,202)	(24,282)	(33,712)
Comprehensive Income [Member] | Subsidiaries [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss), net of tax	(126,906)	(145,511)	(149,550)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(48,802)	10,200	(5,655)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	12,157	8,405	9,694
Other Comprehensive (Income) Loss, Reclassification of Stranded Taxes to Retained Earnings	(27,333)
Other Comprehensive Income (Loss), Net of Tax	(63,978)	18,605	4,039
Accumulated other comprehensive income (loss), net of tax	(190,884)	(126,906)	(145,511)
Energy Related Derivative [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Including Portion Attributable to Noncontrolling Interest [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss), net of tax	0	0	0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	0	0	0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0	0	0
Other Comprehensive (Income) Loss, Reclassification of Stranded Taxes to Retained Earnings	0
Other Comprehensive Income (Loss), Net of Tax	0	0	0
Accumulated other comprehensive income (loss), net of tax	0	0	0
Energy Related Derivative [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Including Portion Attributable to Noncontrolling Interest [Member] | Subsidiaries [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss), net of tax	0	0	0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	0	0	0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0	0	0
Other Comprehensive (Income) Loss, Reclassification of Stranded Taxes to Retained Earnings	0
Other Comprehensive Income (Loss), Net of Tax	0	0	0
Accumulated other comprehensive income (loss), net of tax	0	0	0
Interest Rate Swap [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Including Portion Attributable to Noncontrolling Interest [Member] | Subsidiaries [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss), net of tax	(5,039)	(5,356)	(5,673)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	0	0	0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	385	317	317
Other Comprehensive (Income) Loss, Reclassification of Stranded Taxes to Retained Earnings	1,100
Other Comprehensive Income (Loss), Net of Tax	(715)	317	317
Accumulated other comprehensive income (loss), net of tax	$ (5,754)	$ (5,039)	$ (5,356)