Common Stock and Preferred Stock - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	1 Months Ended					9 Months Ended		1 Months Ended
	Jan. 31, 2013 Director ExecutiveOfficers	Jul. 31, 2012	Sep. 30, 2013	Dec. 31, 2012		Sep. 30, 2013 Restricted Stock Minimum	Sep. 30, 2013 Restricted Stock Maximum	Jan. 31, 2013 Executive officer	Jan. 31, 2013 Executive officer Restricted Stock Minimum	Jan. 31, 2013 Executive officer Restricted Stock Maximum	Jan. 31, 2013 Non-employee directors	Jan. 31, 2013 Non-employee directors Restricted Stock
Class of Stock [Line Items]
Common stock, shares authorized			100,000,000	100,000,000	[1]
Preferred shares, shares authorized			5,000,000	5,000,000	[1]
Total number of shares authorized			105,000,000
Common stock, par value			$ 0.001	$ 0.001	[1]
Preferred shares, par value			$ 0.001	$ 0.001	[1]
Restricted stock issued, shares								285,543			54,227
Restricted stock issued, value								$ 3.6			$ 0.7
Vesting period						1 year	5 years		1 year	3 years
Number of executive officers	2
Number of non-employee directors	7
Shares of restricted stock, vesting date												2014-01
Number of shares repurchased		4,000,000
Common stock price per share		$ 20.00
Value of shares repurchased		80.0
Offering cost of shares repurchased		$ 0.6

[1] Derived from audited financial statements