Employee Benefits ASU Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operations and maintenance	$ 334,883	$ 320,281	$ 315,710
Investment and interest income, net	18,377	38,853	34,797
Miscellaneous non-operating income	12,823	12,051	11,073
Miscellaneous non-operating deductions	11,980	11,580	11,038
Other interest	$ 17,890	18,170	17,509
Accounting Standards Update 2017-07
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operations and maintenance		8,200	7,000
Investment and interest income, net		21,100	20,700
Miscellaneous non-operating income		11,300	9,800
Miscellaneous non-operating deductions		8,400	7,300
Other interest		15,800	16,200
Service cost component		10,800	10,800
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		$ 2,600	$ 3,800