Stockholders' Equity, Stock Repurchase Program and Shelf Registration (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	231 Months Ended
	Mar. 31, 2018	Mar. 31, 2018	Aug. 03, 2017	May 03, 2017	May 04, 2015
Stock Repurchase Program [Abstract]
Incremental Class A shares authorized to buyback (in shares)			0	0
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)	119,334	10,505,200
Average price per share of repurchased shares (in dollars per share)	$ 27.71	$ 43.45
Share available under program to repurchase (in shares)	554,960	554,960