Common Equity - Common Stock Dividends (Details) - $ / shares	3 Months Ended					12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Dividends Paid and Payable [Line Items]
Dividends per share (in dollars per share)		$ 0.52	$ 0.52	$ 0.52	$ 0.52	$ 2.08	$ 1.98	$ 1.74
Subsequent event
Dividends Paid and Payable [Line Items]
Dividends per share (in dollars per share)	$ 0.5525
Annualized dividend (in dollars per share)	$ 2.21
Subsequent event \| Minimum
Dividends Paid and Payable [Line Items]
Target dividend payout ratio (as a percent)	65.00%
Subsequent event \| Maximum
Dividends Paid and Payable [Line Items]
Target dividend payout ratio (as a percent)	70.00%