Changes in Shareholders' Equity (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Dividends declared per share	$ 0.18	$ 0.17
Common Class A [Member]
Dividends declared per share	0.18	0.17
Common Class B [Member]
Dividends declared per share	$ 0.18	$ 0.17
ASC 606 [Member]
Increase decrease in retained earnings			$ 5,600
Increase decrease in non controlling interest			300
ASU 2016-16 [Member]
Increase decrease in retained earnings			500
ASC 842 [Member]
Increase decrease in retained earnings			$ 100