Stockholders’ equity Stockholders equity (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	12 Months Ended
	Mar. 27, 2014	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Common stock, shares authorized (in shares)				100,000,000
Number of shares authorized to be repurchased						1,000,000
Stock repurchased in period (in shares)				0
Treasury Stock, Shares, Acquired		136,756	136,756
Treasury Stock, Value, Acquired, Cost Method		$ 1,000	$ 981
Preferred Stock, Shares Authorized				5,000,000
Preferred Stock, Par or Stated Value Per Share				$ 0.01
Preferred Stock, Shares Outstanding				0
Common stock, par value (in dollars per share)				$ 0.01
Common Stock, Shares, Issued		50,719,884	50,719,884	50,598,901	48,955,909		48,833,323
Common Stock, Shares, Outstanding		50,583,128	50,583,128
Stock Issued During Period, Shares, Acquisitions			20,000	1,477,000
Preferred Share Purchase Right
Class of Stock [Line Items]
Preferred Stock, Par or Stated Value Per Share	$ 0.01
Preferred share purchase rights dividend (shares)	1
Exercise price of rights (USD per share)	$ 30.00