Stock-Based Compensation - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2012
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options expiration term (in years)	10 years
Options and Non Performance Based Grants | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based payment award, award vesting period	3 years
Options and Non Performance Based Grants | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based payment award, award vesting period	4 years
Non Performance Based Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based payment award, award vesting period	5 years
Vesting Percentage of share in year three	33.33%
Vesting Percentage of share in year four	33.33%
Vesting Percentage of share in year five	33.33%
Performance Based Shares Issued in 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based payment award, award vesting period	3 years
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%
Performance Based Shares Issued in 2011 | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares vesting range	0.00%
Performance Based Shares Issued in 2011 | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares vesting range	150.00%
Performance Based Shares Issued Prior to 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based payment award, award vesting period	5 years
Vesting Percentage of share in year three	33.33%
Vesting Percentage of share in year four	33.33%
Vesting Percentage of share in year five	33.33%
Performance Based Shares Issued Prior to 2011 | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares vesting range	0.00%
Performance Based Shares Issued Prior to 2011 | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares vesting range	150.00%