Stockholders' Equity, Stock Repurchase Program and Shelf Registration (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	9 Months Ended	31 Months Ended	203 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Jun. 30, 2018	Nov. 30, 2015	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
Common Class A [Member] | Stock Repurchase Program [Member]
Stock Repurchase Program [Abstract]
Shares repurchased (in shares)	86,333	342,856	1,189,469	9,539,253
Average price per share of repurchased shares (in dollars per share)	$ 25.99	$ 26.52	$ 29.10
Share available under program to repurchase (in shares)	941,949	941,949