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Stock-Based Compensation and Other Benefit Plans - Textual (Details)	1 Months Ended	3 Months Ended		12 Months Ended
	May 31, 2014 USD ($)	Mar. 31, 2019 USD ($)	Jun. 30, 2018	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) $ / shares shares	May 31, 2017 shares	May 31, 2016 shares	May 31, 2015 shares
Options granted (in shares) \| shares				0
Dividend yield				0.00%
Value of options exercised				$ 2,300,000	$ 1,100,000	$ 100,000
Proceeds from options exercised				3,900,000	2,600,000	700,000
Tax benefit from compensation expense				1,500,000	1,700,000	2,300,000
Tax benefits from exercise of stock options				$ 500,000	300,000	100,000
Maximum annual contributions per employee (percent)				50.00%
Employer discretionary contribution amount				$ 3,900,000	$ 1,400,000	$ 900,000
Restricted stock [Member]
Estimated fair value at date of grant (in dollars per share) \| $ / shares				$ 10.38
Number of performance-based restricted stock units issued, at target (in shares) \| shares				135,578
Employee Stock Option [Member]
Vesting period				3 years
Expiration period				10 years
Dividend yield						0.00%
Stock Appreciation Rights (SARs) [Member]
Granted, restricted stock (in shares) \| shares				0
Cash-settled stock appreciation rights outstanding (in shares) \| shares				0	43,000
Performance Based Restricted Stock Units [Member]
Estimated fair value at date of grant (in dollars per share) \| $ / shares				$ 0.00		$ 5.18
Vesting period			3 years
Allocated share-based compensation expense				$ 800,000	$ 1,000,000	$ 1,000,000
Dividend yield						0.00%
Number of performance-based restricted stock units issued, at target (in shares) \| shares				0	0	230,790
Vested (in dollars)				$ 1,900,000
Compensation cost not yet recognized				3,000,000	$ 1,400,000
Range of payout of shares for each executive, minimum						0.00%
Range of payout of shares for each executive, maximum						150.00%
Restricted Stock Units and Restricted Stock Awards [Member]
Allocated share-based compensation expense				7,800,000	8,000,000	$ 8,600,000
Tax benefit from compensation expense				2,800,000	1,900,000	1,500,000
Compensation cost not yet recognized				$ 10,900,000
Period for recognition				1 year 10 months
Fair value of shares vested				$ 11,600,000	$ 10,400,000	$ 3,900,000
Restricted Stock Units and Restricted Stock Awards [Member] \| Minimum [Member]
Vesting period					3 years
Restricted Stock Units and Restricted Stock Awards [Member] \| Maximum [Member]
Vesting period					4 years
Time Based Restricted Stock Units [Member]
Range of payout of shares for each executive, minimum				0.00%
Range of payout of shares for each executive, maximum				150.00%
The 2014 Director Plan [Member]
Number of shares authorized (in shares) \| shares				1,000,000
Shares available for grant (in shares) \| shares				418,680
The 2014 Director Plan [Member] \| Non-employee directors [Member]
Restricted shares granted (in shares)	$ 150,000
The 2014 Director Plan [Member] \| Non-employee directors [Member] \| Restricted stock [Member]
Granted, restricted stock (in shares) \| shares				85,578
Estimated fair value at date of grant (in dollars per share) \| $ / shares				$ 10.75
The 2014 Director Plan [Member] \| Board of Directors Chairman [Member]
Restricted shares granted (in shares)	$ 170,000
The 2015 Plan [Member]
Number of shares authorized (in shares) \| shares							9,800,000	7,800,000	6,000,000
Shares available for grant (in shares) \| shares				1,313,255
The 2015 Plan [Member] \| Full Value Awards [Member]
Shares available for grant reduction rate (in shares)				1.78
The 2015 Plan [Member] \| Grants of Stock Options and Stock Appreciation Rights [Member]
Shares available for grant reduction rate (in shares)				1.00
The 2015 Plan [Member] \| Employees, non-directors [Member]
Plan modification, expense recognized				$ 1,500,000
The 2015 Plan [Member] \| Employees, non-directors [Member] \| Share-based compensation award, tranche one [Member]
Vesting period				90 days
The 2015 Plan [Member] \| Employees, non-directors [Member] \| Share-based compensation award, tranche two [Member]
Vesting period				2 years
Participants Contributions Up to 3 Percent of Compensation [Member]
Employer matching contribution, percent of employees' gross pay				3.00%
Employer matching contribution, percent of match				100.00%
Participants Contributions from 3 Percent to 6 Percent of Compensation [Member]
Options granted (in shares) \| shares				0
Employer matching contribution, percent of match				50.00%
Participants Contributions from 3 Percent to 6 Percent of Compensation [Member] \| Minimum [Member]
Employer matching contribution, percent of employees' gross pay				3.00%
Participants Contributions from 3 Percent to 6 Percent of Compensation [Member] \| Maximum [Member]
Employer matching contribution, percent of employees' gross pay				6.00%
2018 Awards [Member] \| Performance Based Restricted Stock Units [Member]
Allocated share-based compensation expense				$ 1,300,000
2018 Awards [Member] \| Time Based Restricted Stock Units [Member]
Allocated share-based compensation expense				1,300,000
2017 Awards [Member] \| Performance Based Restricted Stock Units [Member]
Allocated share-based compensation expense				1,300,000
2017 Awards [Member] \| Time Based Restricted Stock Units [Member]
Allocated share-based compensation expense				$ 5,300,000
Subsequent event [Member] \| The 2015 Plan [Member] \| Employees, non-directors [Member]
Plan modification, expense recognized		$ 4,200,000