Income Taxes (Details) - USD ($)		3 Months Ended		9 Months Ended	12 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Sep. 30, 2015	Dec. 31, 2012	Dec. 31, 2017
Tax Audits [Abstract]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		21.00%	35.00%
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent		2.40%	2.10%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent	[1]	(4.10%)	0.00%
Effective Income Tax Rate Reconciliation, Tax Cuts and Jobs Act of 2017, Change in Tax Rate, Percent	[2]	2.90%	0.00%
Effective Income Tax Rate Reconciliation, Other Regulatory Items, Percent		(1.50%)	(1.00%)
Effective Income Tax Rate Reconciliation Regulatory Differences Utility Plant Items		(0.70%)	(0.50%)
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent		0.00%	0.50%
Effective Income Tax Rate Reconciliation, Percent		20.00%	36.10%
Unrecognized Tax Benefits [Abstract]
Unrecognized tax benefit - Permanent tax positions		$ 2,400,000				$ 2,300,000
Unrecognized tax benefit - Temporary tax positions		2,000,000				2,000,000
Total unrecognized tax benefit		4,400,000				4,300,000
NOL and tax credit carryforwards		(5,900,000)				(5,900,000)
Upper bound of decrease in unrecognized tax benefit that is reasonably possible		2,000,000
Amounts accrued for penalties related to unrecognized tax benefits		$ 0				$ 0
Internal Revenue Service (IRS)
Tax Audits [Abstract]
Year(s) under examination				2012 and 2013	2010 and 2011
Year of carryback claim under examination					2009
State Jurisdiction (Texas)
Tax Audits [Abstract]
Earliest year subject to examination		2009

[1] The average rate assumption method (ARAM); a method to flow back excess deferred taxes to customers.

[2] As we receive further clarity or direction from our commissions regarding the flow back to customers of excess deferred taxes resulting from the TCJA, the ARAM deferral may decrease during the year, which would result in a reduction to tax expense with a correlating reduction to revenue.