Cash includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the customer may deposit additional funds at any time and effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid Investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months. Short-term investments, exclusive of cash equivalents, generally consist of marketable securities intended to be sold within one year (or the normal operating cycle if longer) and may include trading securities, available-for-sale securities, or held-to-maturity securities (if maturing within one year), as applicable.

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Nature of Business (Details) - USD ($) $ / shares in Units, $ in Thousands				3 Months Ended
	Apr. 04, 2017	Dec. 13, 2016	May 05, 2015	Mar. 31, 2017	Mar. 31, 2016
Nature of Business
Proceeds from issuance of common stock				$ 902	$ 29
Cash, cash equivalents and investments				$ 236,300
IPO \| Common Stock
Nature of Business
Stock sold (in shares)			9,367,708
Share price (in dollars per share)			$ 18.00
Proceeds from issuance of IPO shares			$ 154,800
Follow-on Offering \| Common Stock
Nature of Business
Stock sold (in shares)		5,750,000
Share price (in dollars per share)		$ 25.00
Proceeds from issuance of common stock		$ 134,500
Follow-on Offering \| Common Stock \| Subsequent Event
Nature of Business
Stock sold (in shares)	5,750,000
Share price (in dollars per share)	$ 40.00
Proceeds from issuance of common stock	$ 215,600
Underwriters' Option \| Common Stock
Nature of Business
Stock sold (in shares)		750,000	1,221,874
Underwriters' Option \| Common Stock \| Subsequent Event
Nature of Business
Stock sold (in shares)	750,000