Notes payable, convertible senior notes and line of credit - Summary of Notes Payable (Additional Information) (Detail)	Dec. 31, 2015	Dec. 31, 2014
Debt Disclosure [Abstract]
Average interest rate	1.68%	2.21%

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