Changes in Shareholders' Equity (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Jan. 01, 2020	Dec. 31, 2019	Jan. 01, 2019
Dividends declared per share (in dollars per share)	$ 0.19		$ 0.18	$ 0.38	$ 0.36
Retained earnings	$ 726,233			$ 726,233			$ 698,496
Cumulative Effect, Period of Adoption, Adjustment [Member]
Retained earnings						$ (1,400)		$ 100
Common Class A [Member]
Dividends declared per share (in dollars per share)	$ 0.19	$ 0.19	0.18		0.18
Common Class B [Member]
Dividends declared per share (in dollars per share)	$ 0.19	$ 0.19	$ 0.18		$ 0.18