401(k) savings plan (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
401(k) savings plan [Abstract]
Matching of qualified deferrals by employer, maximum (in hundredths)	50.00%
Percentage of employee's gross salary subject to employer's matching contribution (in hundredths)	6.00%
Matching contributions made by employer	$ 2.4	$ 2.0	$ 1.9

Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

No definition available.

Name:	us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration