Stock-based Compensation - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended					3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Equity Incentive Plan 2009 [Member]	Dec. 31, 2012 Equity Incentive Plan 2009 [Member]	Dec. 31, 2011 Equity Incentive Plan 2009 [Member]	Mar. 31, 2013 Restricted stock units [Member]	Mar. 31, 2013 Restricted stock units [Member] Officers and employees [Member]	Mar. 31, 2012 Restricted stock units [Member] Officers and employees [Member]	Mar. 31, 2012 Restricted stock units [Member] Officers [Member]	Mar. 31, 2013 Restricted stock units [Member] Directors [Member]	Mar. 31, 2012 Restricted stock units [Member] Directors [Member]	Mar. 31, 2013 Performance stock units [Member]	Mar. 31, 2013 Performance stock units [Member] Officers [Member]	Mar. 31, 2013 Stock options [Member]	Mar. 31, 2012 Stock options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Additional shares of common stock				1,250,000	1,219,787
Shares available for future issuance			553,807
Number of stock options, granted	80,930													80,930	92,500
Intrinsic value of stock options exercised														$ 1.6	$ 3.2
Cash received from stock options exercised														0.2	0.9
Unrecognized stock-based compensation expense related to unvested options						43.0						8.5		9.0
Weighted average period of compensation expense to be recognized						1 year 10 months 17 days						2 years 22 days		1 year 4 months 17 days
Number of restricted stock units granted						1,386,295	1,378,448	311,100	16,003	7,847	12,234	382,500	382,500
Number of vesting period							4 years					4 years
Additional paid-in capital pool related to tax windfall benefits from stock option exercises	9.0
Net operating loss carryforwards but not reflected in deferred tax assets	$ 55.5	$ 77.6