Stock Option Plans, Stock Grants, Stock Appreciation Rights and Employee Stock Purchase Plan - Additional Information (Detail) (USD $)	12 Months Ended																								12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011 Performance Based Shares	Dec. 31, 2010 Performance Based Shares	Dec. 31, 2012 Maximum	Dec. 31, 2012 Options and Non Performance Based Grants Minimum	Dec. 31, 2012 Options and Non Performance Based Grants Maximum	Dec. 31, 2012 Non Performance Based Shares	Dec. 31, 2012 Performance Based Shares Issued in 2011	Dec. 31, 2012 Performance Based Shares Issued in 2011 Minimum	Dec. 31, 2012 Performance Based Shares Issued in 2011 Maximum	Dec. 31, 2012 Performance Based Shares Issued Prior to 2011	Dec. 31, 2012 Performance Based Shares Issued Prior to 2011 Minimum	Dec. 31, 2012 Performance Based Shares Issued Prior to 2011 Maximum	Dec. 31, 2012 Stock appreciation rights	Dec. 31, 2011 Stock appreciation rights	Dec. 31, 2010 Stock appreciation rights	Dec. 31, 2012 Stock appreciation rights Minimum	Dec. 31, 2012 Employee stock purchase plan	Dec. 31, 2012 Restricted stock units	Dec. 31, 2011 Restricted stock units	Dec. 31, 2010 Restricted stock units	Dec. 31, 2005 2005 Stock Incentive Plan	Dec. 31, 2012 Amended And Restated 2005 Stock Incentive Plan	Apr. 30, 2009 Amended And Restated 2005 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of stock authorized for issuance																300,000								1,000,000		1,500,000
Stock issued during period under incentive plan																									1,225,805
Number of stock available to be issued																									274,195
Share based payment award, award vesting period							3 years	4 years	5 years	3 years			5 years
Options expiration term (in years)						10 years
Vesting Percentage of share in year three									33.33%				33.33%
Vesting Percentage of share in year four									33.33%				33.33%
Vesting Percentage of share in year five									33.33%				33.33%
Percentage of shares vesting over the period										50.00%
Percentage of remaining shares vesting over the period dependent on compound annual growth book value per share immediately prior to vesting date										50.00%
Percentage of shares vesting range											0.00%	150.00%		0.00%	150.00%
Performance based shares that vested				22,933	8,333
Fair value of total vested shares	$ 4,600,000	$ 8,600,000	$ 7,800,000
Weighted average grant date fair value of all RSU granted																					$ 48.21	$ 50.06	$ 39.54
Unrecognized compensation expense, net of estimated forfeitures																					11,900,000	4,900,000
Weighted Average Periods																					2 years 2 months 12 days	2 years 8 months 12 days
Aggregate fair value of all unvested RSU																					30,000,000	25,100,000
Exercise price as percentage of fair market value																			90.00%	90.00%
Percentage of employees base compensation through payroll that can be invested for the purchase of common stock, maximum	10.00%
Number of shares which can be acquired by employee, maximum	1,000
Total market value of shares that can be acquired by employee	$ 25,000
Employee Stock Purchase Plan, shares to be purchased in following period	8,136
Employee Stock Purchase Plan, shares purchased by employees	16,000	19,000	22,000
Discount on employee stock purchase plan	10.00%