Common Stock - Additional Information (Detail) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	May 15, 2012
Class of Stock [Line Items]
Common shares authorized for issuance	1,000,000	1,000,000	1,100,000
Percentage of shares owned by funds affiliated with VantagePoint	20.40%
Minimum
Class of Stock [Line Items]
Percentage of outstanding voting stocks VantagePoint should own to have access to books and records	10.00%
Percentage of outstanding shares of common stock VantagePoint should own to require some actions by Company	25.00%
If VantagePoint owns between 20% and 50% of outstanding shares
Class of Stock [Line Items]
Number of directors to be designated, removed or replaced by VantagePoint	2
If VantagePoint hold at least 10% of outstanding shares
Class of Stock [Line Items]
Number of directors to be selected to serve on compensation committee, nominating and corporate governance committee and any other special committee of board of directors	1