Stockholders' Equity, Stock Repurchase Program and Shelf Registration (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	6 Months Ended	31 Months Ended	203 Months Ended	234 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Jun. 30, 2018	Nov. 30, 2015	Jun. 30, 2018	Apr. 23, 2018
Shelf Registration [Abstract]
Maximum amount of debt and equity to be issued under shelf registration						$ 500
Amount available for debt and equity issuance under shelf registration	$ 500	$ 500	$ 500		$ 500
Common Class A [Member] | Stock Repurchase Program [Member]
Stock Repurchase Program [Abstract]
Shares repurchased (in shares)	137,189	256,523	1,103,136	9,539,253	10,642,389
Average price per share of repurchased shares (in dollars per share)	$ 25.81	$ 26.70	$ 29.35		$ 43.22
Share available under program to repurchase (in shares)	417,771	417,771	417,771		417,771