Stock-based compensation - Narrative (Details) $ / shares in Units, $ in Thousands				1 Months Ended		12 Months Ended
	Jan. 01, 2017	Jan. 30, 2016	Dec. 18, 2014 USD ($) installment $ / shares shares	Feb. 28, 2011 shares	Apr. 30, 2010 shares	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($)	Dec. 31, 2015 $ / shares shares	Dec. 31, 2014 $ / shares	Jun. 30, 2019 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Remitted cash to appropriate tax authorities | $						$ 733	$ 954	$ 998
Stock Appreciation Units (SARs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options to purchase shares, outstanding (in shares)						163,471	192,872
Intrinsic value of options exercised | $						$ 100	$ 100	200
Number of days for stock price to be traded above mentioned price						20 days
Minimum closing stock price (USD per share) | $ / shares						$ 15.00
Share-based compensation expense (gain) | $						$ 200	(100)
Stock grants vested value | $						$ 0	$ 0
Employee stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options to purchase shares, outstanding (in shares)						2,598,745	3,202,745
Shares available for future grant (in shares)						1,309,215	1,102,323
Authorized for issuance (in shares)						1,623,244
Unrecognized stock-based compensation expense | $						$ 800
Fair value of options vested | $						1,600	$ 2,300	1,500
Intrinsic value of options exercised | $						$ 1,300	$ 2,400	$ 3,000
Weighted-average fair value of options granted (USD per share) | $ / shares						$ 2.68	$ 4.03	$ 4.43
Remaining weighted-average period						1 year 2 months 12 days
Stock Option and Restricted Stock Unit Activity
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units (in shares)						600,000
Weighted-average fair value granted (USD per share) | $ / shares										$ 4.72
Number of days for stock price to be traded above mentioned price						20 days
Minimum closing stock price (USD per share) | $ / shares						$ 15.00
Share-based compensation expense (gain) | $									$ 4,800
Stock Option and Restricted Stock Unit Activity | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average fair value granted (USD per share) | $ / shares											$ 1.65
Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares outstanding (in shares)						3,170,981	2,486,028
Unrecognized stock-based compensation expense | $						$ 12,300
Remaining weighted-average period						2 years 4 months 24 days
Restricted stock units (in shares)						1,974,019
Weighted-average fair value granted (USD per share) | $ / shares						$ 4.90	$ 6.83	$ 7.86
Stock grants vested value | $						$ 8,900	$ 9,800	$ 7,600
Aggregate intrinsic value, vested | $						$ 4,700	$ 7,400	$ 6,400
Common stock withheld shares (in shares)						155,267	147,572	126,999
Remitted cash to appropriate tax authorities | $						$ 700	$ 1,000	$ 1,000
Vested (in shares)						1,018,143
Market-based Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards expiration period from date of grant						7 years
Remaining weighted-average period						1 year
Restricted stock units (in shares)						10,000	695,000
Weighted-average fair value granted (USD per share) | $ / shares						$ 4.86	$ 5.82
Canceled during period						63,500	0
Vested (in shares)						0	0
Employee service share-based compensation, nonvested awards, compensation not yet recognized, stock options | $						$ 1,300
Remaining contractual term						5 years 7 months 6 days
Stock Appreciation Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options to purchase shares, outstanding (in shares)						200,000
Liability for settlement | $						$ 800	$ 600
2004 Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options to purchase shares, outstanding (in shares)						65,644
Shares available for future grant (in shares)						0
2007 Stock Appreciation Grants Plan | Stock Appreciation Units (SARs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares outstanding (in shares)						13,471
2010 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grant (in shares)						1,093,354
Shares outstanding (in shares)						5,831,720
Awards expiration period from date of grant						10 years
Maximum aggregate number of shares of common stock that may be issued (in shares)					865,420	8,000,000
Number of shares of common stock outstanding increase percentage					3.50%
2010 Equity Incentive Plan | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price, percentage of fair value of common stock on grant date						100.00%
Options vesting period						3 years
2010 Equity Incentive Plan | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options vesting period						4 years
2010 Equity Incentive Plan | Nonqualified Stock Options | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price, percentage of fair value of common stock on grant date						85.00%
2010 ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grant (in shares)						1,717,211
Maximum aggregate number of shares of common stock that may be issued (in shares)				342,568		600,000						1,500,000
Number of shares of common stock outstanding increase percentage				3.50%
Offering period						12 months
2011 Inducement Award Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grant (in shares)						215,861
Shares outstanding (in shares)						513,862
Exercise price, percentage of fair value of common stock on grant date				100.00%
Awards expiration period from date of grant				10 years
Maximum aggregate number of shares of common stock that may be issued (in shares)				750,000
Authorized for issuance (in shares)										100,000
2014 Repricing Offer | Stock Appreciation Units (SARs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued under repricing offer (in shares)			87,354
2014 Repricing Offer | Stock Options and Stock Appreciation Rights
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued under repricing offer (in shares)			2,373,692
Unrecognized stock-based compensation expense | $			$ 2,600
Incremental compensation cost | $			$ 900
Exercise price per share (USD per share) | $ / shares			$ 3.50
Percentage of shares subject to repriced eligible stock			50.00%
Number of monthly installments | installment			12
Remaining vesting term			60 days
Period prior to expiration date when repriced awards vested and become exercisable			60 days
2014 Repricing Offer | Stock Options and Stock Appreciation Rights | Vesting In 12 Equal Monthly Installments
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares subject to repriced eligible stock			50.00%
2014 Repricing Offer | Stock Options and Stock Appreciation Rights | 60th Day Prior to Expiration Date
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares subject to repriced eligible stock	50.00%	100.00%
2014 Repricing Offer | Employee stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued under repricing offer (in shares)			1,948,631