Capital Stock, Repurchases of Common Stock (Details) - USD ($) $ in Thousands, shares in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jul. 31, 2018	Oct. 31, 2015	Aug. 31, 2013	Dec. 31, 1998
Repurchases of Common Stock [Abstract]
Class A common stock repurchased	$ 69,565	$ 71,731	$ 247,208
1998 Stock Repurchase Plan [Member]
Repurchases of Common Stock [Abstract]
Authorized amount							$ 10,000
Increase in authorized amount						$ 400,000
2015 Stock Repurchase Plan [Member]
Repurchases of Common Stock [Abstract]
Authorized amount					$ 500,000
2018 Stock Repurchase Plan [Member]
Repurchases of Common Stock [Abstract]
Authorized amount				$ 500,000
Amount available for repurchases	471,000
Treasury Stock [Member]
Repurchases of Common Stock [Abstract]
Class A common stock repurchased	$ 69,565	$ 71,731	$ 247,208
Treasury Stock [Member] | 2015 Stock Repurchase Plan [Member]
Repurchases of Common Stock [Abstract]
Class A common stock repurchased (in shares)	0.5	1.2	4.5
Class A common stock repurchased	$ 40,600	$ 71,700	$ 247,200
Treasury Stock [Member] | 2018 Stock Repurchase Plan [Member]
Repurchases of Common Stock [Abstract]
Class A common stock repurchased (in shares)	0.4
Class A common stock repurchased	$ 29,000