Stock-Based Compensation - Narrative (Details) - USD ($)			12 Months Ended
	Apr. 24, 2017	Apr. 01, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock-Based Compensation
Tax benefit from compensation expense			$ 2,000,000	$ 4,000,000	$ 4,000,000
Unrecognized compensation expense			$ 14,000,000
Future forfeiture rate (as a percent)			2.00%
Period for recognition of compensation cost			2 years
Aggregate intrinsic value of stock options outstanding			$ 9,000,000
Aggregate intrinsic value of stock options exercisable			$ 9,000,000
Class A common stock
Stock-Based Compensation
Number of common stock available for future grant under stock incentive plans (in shares)			8,000,000
Employee And Non Employee Stock Option
Stock-Based Compensation
Shares outstanding under stock incentive plans (in shares)			5,013,038	4,951,256	5,968,763	5,893,241
Granted (in shares)			215,500	1,262,500	732,000
Tax benefits from stock options exercised			$ 400,000	$ 3,000,000	$ 2,000,000
Options exercised in period intrinsic value			$ 2,000,000	20,000,000	8,000,000
Employee Stock Options
Stock-Based Compensation
Maximum expiration term			10 years
Vesting period			5 years
Granted (in shares)		1,100,000
Nonvested options forfeited (in shares)	600,000
Non-Employee Stock Option
Stock-Based Compensation
Maximum expiration term			5 years
Restricted stock units
Stock-Based Compensation
Equity instruments other than options vested in period			$ 0	$ 200,000	$ 2,000,000