Income Taxes Deferred Tax Liability (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Deferred Tax Assets, [Abstract]
NOL carryforward	$ 72,964	$ 3,677
Pension / postretirement benefits	45,847	54,440
Compensation accruals	18,715	17,441
Production tax credit	17,034	6,550
Customer advances	15,837	14,197
AMT credit carryforward	13,599	13,143
Unbilled revenue	12,743	28,390
Environmental liability	9,698	9,410
Interest rate hedges	7,192	6,483
Property taxes	3,767	24,650
Regulatory liabilities	2,290	2,862
Reserves and accruals	1,121	0
QF obligations	1,025	2,636
Other, net	3,173	3,696
Deferred Tax Asset	225,005	187,575
Deferred Tax Liabilities, [Abstract]
Excess tax depreciation	(459,588)	(392,113)
Goodwill amortization	(168,165)	(152,065)
Flow through depreciation	(160,604)	(125,441)
Regulatory assets	(12,230)	(14,901)
Reserves and accruals	0	(4,587)
Deferred Tax Liability	(800,587)	(689,107)
Deferred Tax Liability, net	$ (575,582)	$ (501,532)