Stock based compensation (Details 3) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013
Restricted stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares	129
Weighted-average grant date fair value, awarded	$ 40.73
Aggregate fair value	$ 5,254
Restricted stock [Member] | MinimumMember
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	1 year
Restricted stock [Member] | MaximumMember
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	4 years
Restricted stock units (RSU) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares	16
Weighted-average grant date fair value, awarded	$ 40.66
Aggregate fair value	661
Restricted stock units (RSU) [Member] | MinimumMember
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	4 years
Restricted stock units (RSU) [Member] | MaximumMember
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	4 years
StockOptionsMember
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares	109
Weighted-average grant date fair value, awarded	$ 7.83
Aggregate fair value	$ 849
StockOptionsMember | MinimumMember
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	1 year
StockOptionsMember | MaximumMember
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	5 years