Stockholders' equity (Details) $ / shares in Units, $ in Millions			12 Months Ended
	Jul. 01, 2015 shares	Jun. 12, 2015 USD ($) $ / shares	Dec. 31, 2015 shares	Dec. 31, 2017 class_of_stock shares	Dec. 31, 2016 shares
Equity [Abstract]
Number of classes of stock authorized | class_of_stock				2
Common stock, shares authorized (in shares)	300,000,000			300,000,000	300,000,000
Preferred stock, shares authorized (in shares)	10,000,000			10,000,000	10,000,000
Class of Stock [Line Items]
Common stock, shares issued (in shares)				47,215,720	46,172,318
Common stock, shares outstanding (in shares)				47,202,310	46,142,483
Preferred stock, shares issued (in shares)				0	0
Common stock votes per share				1
Preferred stock, shares outstanding (in shares)				0	0
Common Stock and Series A Preferred Stock
Class of Stock [Line Items]
Dividends declared, per share (USD per share) | $ / shares		$ 0.36368
Series B and Series B-1 Preferred Stock
Class of Stock [Line Items]
Dividends declared, per share (USD per share) | $ / shares		$ 0.72736
Dividends declared and paid to common stockholders | $		$ 20.0
Common Stock
Class of Stock [Line Items]
Issuance of common stock from initial public offering, net of issuance costs (in shares)			7,525,000
IPO | Common Stock
Class of Stock [Line Items]
Shares issued upon conversion of redeemable convertible preferred stock (in shares)	35,017,884
Issuance of common stock from initial public offering, net of issuance costs (in shares)	7,525,000