Share-Based Compensation - Summary of Option Activity (Details) - $ / shares	6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Shares (In thousands)
Outstanding, beginning balance (in shares)	2,355,787	5,477,617	5,477,617
Granted (in shares)	0	0
Exercised (in shares)	(1,092,789)	(1,735,000)
Forfeited or expired (in shares)	(179,000)	(91,000)
Outstanding, ending balance (in shares)	1,083,998.00	3,651,617	2,355,787	5,477,617
Weighted-Average Exercise Price
Outstanding, beginning balance (in USD per share)	$ 9.81	$ 9.72	$ 9.72
Granted (in USD per share)	0.00	0.00
Exercised (in USD per share)	9.81	9.65
Forfeited or expired (in USD per share)	9.81	9.81
Outstanding, ending balance (in USD per share)	$ 9.81	$ 9.76	$ 9.81	$ 9.72
Weighted-Average Remaining Contractual Terms (Years)
Outstanding	6 years 4 months 24 days	5 years 6 months	5 years 4 months 24 days	6 years
Exercisable
Shares (in shares)	332,000	3,111,120
Weighted-Average Exercise Price (in USD per share)	$ 9.81	$ 9.72
Weighted-Average Remaining Contractual Terms (Years)	4 years 4 months 24 days	4 years 11 months