Notes payable, convertible senior notes and line of credit - Summary of Notes Payable (Additional Information) (Detail)	Dec. 31, 2014	Dec. 31, 2013
Debt Disclosure [Abstract]
Average interest rate	2.21%us-gaap_DebtWeightedAverageInterestRate	2.82%us-gaap_DebtWeightedAverageInterestRate

Weighted average interest rate of debt outstanding.

No definition available.

Name:	us-gaap_DebtWeightedAverageInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant