Stock incentive plans (Details 5)	9 Months Ended
	Sep. 30, 2015 installment $ / shares shares
RSU
Stock incentive plans
Shares of common stock issued resulting from vesting	498,391
Number of Shares
Non-vested at beginning of period (in shares)	1,385,000
Granted (in shares)	1,423,000
Vested (in shares)	(748,532)
Forfeited (in shares)	(52,000)
Non-vested at end of period (in shares)	2,007,000
Weighted Average Grant-Date Fair Value
Non-vested at beginning period (In dollars per share) | $ / shares	$ 6.09
Granted (In dollars per share) | $ / shares	7.57
Vested (In dollars per share) | $ / shares	8.01
Canceled/forfeited (In dollars per share) | $ / shares	7.03
Non-vested at end of period (In dollars per share) | $ / shares	$ 6.40
Time-based restricted stock unit awards | Executive And Non Executive Member | Minimum
Stock incentive plans
Vesting period	2 years
Time-based restricted stock unit awards | Executive And Non Executive Member | Maximum
Stock incentive plans
Vesting period	3 years
Time-based restricted stock unit awards | Non-employee directors and existing members
Stock incentive plans
Vesting period	1 year
Time-based restricted stock unit awards | Non-employee directors and new 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	33.33%
Continuous service period of individual	12 months
Number of equal quarterly installments for remaining award vesting | installment	8