Stock incentive plans (Details 6) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
RSUs
Number of Shares
Nonvested at the beginning of the period (in shares)	753,000
Granted (in shares)	1,650,000	799,000
Vested (in shares)	(868,000)	(6,000)
Canceled/forfeited (in shares)	(150,000)	(40,000)
Nonvested at the end of the period (in shares)	1,385,000	753,000
Weighted Average Grant-Date Fair Value
Nonvested at the beginning of the period (In dollars per share)	$ 6.22
Granted (In dollars per share)	$ 6.02	$ 6.21
Vested (In dollars per share)	$ 6.09	$ 6.39
Canceled/forfeited (In dollars per share)	$ 6.02	$ 6.05
Nonvested at the end of the period (In dollars per share)	$ 6.09	$ 6.22
Total stock-based compensation expense	$ 6,901
Weighted average period over which unrecognized compensation expense is expected to be recognized	2 years 1 month 6 days
Time-based restricted stock unit awards
Weighted Average Grant-Date Fair Value
Shares vested	868,211
Shares issued	581,866
Time-based restricted stock unit awards | Executive and non executive personnel | Minimum
Stock incentive plans
Vesting period	2 years
Time-based restricted stock unit awards | Executive and non executive personnel | Maximum
Stock incentive plans
Vesting period	3 years
Time-based restricted stock unit awards | Existing board members
Stock incentive plans
Vesting period	1 year
Time-based restricted stock unit awards | New board members
Stock incentive plans
Vesting period	4 years
Vesting percentage when the individual completes 12 months of continuous service	25.00%
Performance-based restricted stock unit awards
Stock incentive plans
Vesting percentage when the individual completes 12 months of continuous service	33.00%
Continuous service period of individual	12 months
Number of equal quarterly installments for remaining award vesting	8