Common Stock (Antidilutive Shares) (Details) - USD ($) $ in Millions			12 Months Ended
		Jan. 30, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restricted Stock
Common Stock [Line Items]
Antidilutive Securities Excluded from Computation of Diluted Earnings Per Share, Amount			62,836	67,739	53,703
Performance Shares
Common Stock [Line Items]
Performance Shares Payout Level			100.00%	100.00%	100.00%
Antidilutive Securities Excluded from Computation of Diluted Earnings Per Share, Amount	[1]		22,815	0	47,246
Performance shares payout based upon performance			0	0	0
Maximum
Common Stock [Line Items]
Performance Shares Payout Level			200.00%
Subsequent Event | NMPRC Case No. 19-00033-UT | Maximum
Common Stock [Line Items]
Proceeds from issuance of common stock, requested		$ 200.0
FERC | Subsequent Event | Maximum
Common Stock [Line Items]
Proceeds from issuance of common stock, requested		$ 200.0

[1] Certain performance shares were excluded from the computation of diluted earnings per share as no payouts would have been required based upon performance at the end of each corresponding period.