Stockholders' Equity - Shares Outstanding (Details) - $ / shares	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Treasury stock (in shares)	14,592,888		14,592,888
Preferred stock, shares authorized (in shares)	50,000,000	50,000,000
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01	$ 0.01
Preferred stock, shares issued (in shares)	0		0
Preferred stock, shares outstanding (in shares)	0		0
Class A
Class of Stock [Line Items]
Common stock, shares authorized (in shares)	175,000,000	135,000,000
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01	$ 0.01
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued at beginning of period (in shares)	71,675,379	66,033,509
Issuance (repurchased) pursuant to benefit plans (in shares)	123,216	(15,906)
Restricted stock forfeited and withheld for taxes (in shares)	(60,081)	(7,590)
Stock options exercised (in shares)	8,438	(6,975)
Shares issued at end of period (in shares)	71,746,952	66,003,038
Treasury stock (in shares)	14,592,888	14,592,888
Total shares outstanding (in shares)	57,154,064	51,410,150
Class B
Class of Stock [Line Items]
Common stock, shares authorized (in shares)	25,000,000
Common stock, par value (in dollars per share)	$ 0.01		$ 0.01
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued at beginning of period (in shares)	23,793,669
Issuance (repurchased) pursuant to benefit plans (in shares)	0
Restricted stock forfeited and withheld for taxes (in shares)	0
Stock options exercised (in shares)	0
Shares issued at end of period (in shares)	23,793,669
Treasury stock (in shares)	0
Total shares outstanding (in shares)	23,793,669