Income Taxes (Details) - USD ($)		3 Months Ended			12 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Sep. 30, 2015	Dec. 31, 2016	Dec. 31, 2012	Dec. 31, 2017
Income Tax Examination [Line Items]
Unrecognized Tax Benefits, Income Tax Penalties Accrued		$ 0					$ 0
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		4.90%	4.00%
Effective Income Tax Rate Reconciliation, Tax Credit, Percent		(6.00%)	(4.00%)
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent	[1]	(5.80%)	(0.10%)
Effective Income Tax Rate Reconciliation, Tax Cuts and Jobs Act of 2017, Change in Tax Rate, Percent	[2]	5.40%	0.00%
Effective Income Tax Rate Reconciliation, Other Regulatory Items, Percent		(1.00%)	(0.50%)
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent		(1.60%)	(1.50%)
Effective Income Tax Rate Reconciliation, Percent		16.90%	32.90%
Unrecognized Tax Benefits [Abstract]
Unrecognized tax benefit — Permanent tax positions		$ 21,000,000					20,000,000
Unrecognized tax benefit — Temporary tax positions		19,000,000					19,000,000
Total unrecognized tax benefit		40,000,000					39,000,000
NOL and tax credit carryforwards		(32,000,000)					$ (31,000,000)
Upper bound of decrease in unrecognized tax benefit that is reasonably possible		$ 26,000,000
Internal Revenue Service (IRS)
Tax Audits [Abstract]
Year(s) under examination				2012 and 2013		2010 and 2011
Year of carryback claim under examination						2009
Colorado
Tax Audits [Abstract]
Earliest year subject to examination		2009
Minnesota
Tax Audits [Abstract]
Year(s) under examination					2010 through 2014
Earliest year subject to examination		2009
Texas
Tax Audits [Abstract]
Earliest year subject to examination		2009
Wisconsin
Tax Audits [Abstract]
Year(s) under examination					2012 and 2013
Earliest year subject to examination		2012

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

[2] As we receive direction from our regulatory commissions regarding the return of excess deferred taxes (to our customers resulting from the TCJA), the ARAM deferral may decrease during the year, which would result in a reduction to tax expense with a corresponding reduction to revenue.