Stock Incentive Plans - Stock Option Activity (Details) - Stock Option - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Number Of Shares (in thousands)
Outstanding, beginning balance (in shares)	1,636
Options exercised (in shares)	307
Options forfeited (in shares)	17
Options granted (in shares)	203
Outstanding, end balance (in shares)	1,515	1,800	1,636
Exercisable, end of period (in shares)	959
Vested and expected to vest, end of period (in shares)	1,494
Weighted Average Exercise Price
Outstanding, beginning balance (in usd per share)	$ 18.94
Options exercised (in usd per share)	16.72
Options forfeited (in usd per share)	24.37
Options granted (in usd per share)	32.45
Outstanding, ending balance (in usd per share)	21.14		$ 18.94
Exercisable, end of period (in usd per share)	18.85
Vested and expected to vest, end of period (in usd per share)	$ 21.07
Other Disclosures
Outstanding, weighted-average remaining contractual term	4 years 2 months 19 days		3 years 9 months 11 days
Exercisable, weighted-average remaining contractual term	3 years 22 days
Vested and expected to vest, weighted-average remaining contractual term	4 years 2 months 9 days
Outstanding, aggregated intrinsic value	$ 12,522		$ 21,597
Exercisable, aggregated intrinsic value	9,663
Vested and expected to vest, aggregated intrinsic value	$ 12,429