Stock incentive plans (Details 5) - 6 months ended Jun. 30, 2015	installment $ / shares shares
RSU
Number of Shares
Nonvested at December 31, 2014 (in shares)	1,385,000
Granted (in shares)	1,267,000
Vested (in shares)	(531,000)
Forfeited (in shares)	(34,000)
Nonvested at June 30, 2015 (in shares)	2,087,000
Weighted Average Grant-Date Fair Value
Nonvested at December 31, 2014 (In dollars per share) | $ / shares	$ 6.09
Granted (In dollars per share) | $ / shares	7.40
Vested (In dollars per share) | $ / shares	8.18
Canceled/forfeited (In dollars per share) | $ / shares	7.14
Nonvested at June 30, 2015 (In dollars per share) | $ / shares	$ 6.34
Time-based restricted stock unit awards
Stock incentive plans
Shares issued, net of share withheld for income taxes	360,465
Shares vested	531,488
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	33.00%
Continuous service period of individual	12 months
Number of equal quarterly installments for remaining award vesting | installment	8