Common Equity - Common Stock Dividends (Details) - $ / shares	3 Months Ended					12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Dividends Paid and Payable [Line Items]
Dividends per share (in dollars per share)		$ 0.5525	$ 0.5525	$ 0.5525	$ 0.5525		$ 2.21	$ 2.08	$ 1.98
Subsequent event
Dividends Paid and Payable [Line Items]
Dividends per share (in dollars per share)	$ 0.59
Annualized dividend (in dollars per share)						$ 2.36
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%