Stock incentive plan (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options outstanding, beginning of period (in shares)	2,929	6,830	6,210
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross	0	0	884
Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures in Period	(592)	(3,901)	(264)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period	0	0	0
Options outstanding, end of period (in shares)	2,337	2,929	6,830
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number	2,337	1,730	5,318
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Options outstanding, beginning of period (in dollars per share)	$ 24.47	$ 92.34	$ 99.03
Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price	0	0	25.28
Share-based Compensation Arrangements by Share-based Payment Award, Options, Forfeitures in Period, Weighted Average Exercise Price	25.48	168.79	25.28
Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price	0	0	0
Options outstanding, end of period (in dollars per share)	24.21	24.47	92.34
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Options outstanding, beginning of the period, fair value (in dollars per share)	12.71	57.97	64.96
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value	0	0	15.52
Share Based Compensation Arrangement By Share Based Payment Award Options Forfeited Fair Value	12.53	95.85	80.33
Share Based Compensation Arrangement By Share Based Payment Award Options Exercised During Period Fair Value	0	0	0
Options outstanding, ending balance fair value (in dollars per share)	$ 12.76	$ 12.71	$ 57.97
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value	$ 0	$ 0	$ 0