Average Market Price of Common Stock and the Incremental Shares that were Dilutive or Potentially Dilutive (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Average market price of Signature common stock	$ 8.77	$ 11.89	$ 9.93	$ 7.76
Dilutive and potentially dilutive common stock equivalents:
Total dilutive and potentially dilutive common stock equivalents	753,876	1,426,703	958,601	624,814
Unvested Restricted Common Stock
Dilutive and potentially dilutive common stock equivalents:
Total dilutive and potentially dilutive common stock equivalents	47,909	275,151	43,213	233,483
Common Stock Options
Dilutive and potentially dilutive common stock equivalents:
Total dilutive and potentially dilutive common stock equivalents	377,260	514,924	449,050	215,406
Warrants
Dilutive and potentially dilutive common stock equivalents:
Total dilutive and potentially dilutive common stock equivalents	328,707	636,628	466,338	175,925