Employee Benefits ASU Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operations and maintenance	$ 80,160	$ 79,187	$ 321,254	$ 317,518
Investment and interest income, net	5,155	9,263	34,745	35,959
Miscellaneous non-operating income	3,136	2,895	12,292	10,983
Miscellaneous non-operating deductions	2,743	2,828	11,494	11,513
Other interest	$ 4,654	4,345	$ 18,479	17,155
Accounting Standards Update 2017-07
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operations and maintenance		2,100		7,500
Investment and interest income, net		5,300		20,800
Miscellaneous non-operating income		2,800		10,300
Miscellaneous non-operating deductions		2,100		7,500
Other interest		3,900		16,000
Service Cost Component		2,900		11,000
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		$ 800		$ 3,500