Stock-Based Compensation, Stock Option Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Stock Options Shares Outstanding, Beginning balance (in shares)	5,281
Granted	1,050
Exercised	(355)
Forfeited or expired	(72)
Stock Options Shares Outstanding, Ending balance (in shares)	5,905	5,281
Options exercisable (in shares)	3,691
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Stock Options Outstanding, Weighted Average Exercise Price, Beginning balance (in usd per share)	$ 19.09
Granted, Weighted Average Exercise Price	22.77
Exercised, Weighted Average Exercise Price	15.61
Forfeited or expired, Weighted Average Exercise Price	23.46
Stock Options Outstanding, Weighted Average Exercise Price, Ending balance (in usd per share)	19.91	$ 19.09
Options Exercisable, Weighted Average Exercise Price (in usd per share)	$ 18.01
Stock Options Outstanding, Weighted Average Remaining Contractual Term	6 years 9 months 25 days	6 years 2 months 4 days
Options exercisable, Weighted Average Remaining Contractual Term	5 years 6 months 3 days
Stock Options Outstanding, Aggregate Intrinsic Value	$ 15,870	$ 12,392
Options exercisable, Aggregate Intrinsic Value	$ 14,953