Shareholders' Equity (Common And Equity Stock) (Details) - USD ($) $ / shares in Units, $ in Thousands, shares in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Shareholders' Equity [Abstract]
Distributions paid to common shareholders	$ 23,200	$ 20,300	$ 69,364	$ 60,932
Dividends paid per common share	$ 0.85	$ 0.75	$ 2.55	$ 2.25	$ 0.75
Equity stock, shares authorized	100.0		100.0