Employee Benefits ASU Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operations and maintenance	$ 83,355	$ 74,685	$ 252,370	$ 236,145	$ 336,506	$ 315,677
Investment and interest income, net	16,815	8,989	33,042	30,308	41,587	39,281
Miscellaneous non-operating income	3,037	3,075	9,245	8,867	12,428	11,901
Miscellaneous non-operating deductions	3,263	3,180	8,775	8,677	11,677	11,542
Other interest	$ 4,127	4,673	$ 13,896	13,746	$ 18,320	17,757
Accounting Standards Update 2017-07
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operations and maintenance		2,100		6,200		8,500
Investment and interest income, net		5,300		15,800		21,000
Miscellaneous non-operating income		2,800		8,500		11,300
Miscellaneous non-operating deductions		2,100		6,300		8,200
Other interest		3,900		11,800		15,600
Service cost component		2,700		8,100		10,700
Net periodic benefit cost previously reported in operating income (including non-service cost components)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operations and maintenance		$ 600		$ 1,900		$ 2,200