Stock-Based Compensation - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended					3 Months Ended		6 Months Ended
	Jun. 30, 2013 Market Based Awards	Jun. 30, 2012 Market Based Awards	Jun. 30, 2013 Market Based Awards	Jun. 30, 2012 Market Based Awards	Jun. 30, 2013 Performance Shares	Jun. 30, 2012 Performance Shares	Jun. 30, 2013 Performance Shares	Jun. 30, 2012 Performance Shares	Mar. 31, 2013 Service Based Award	Jun. 30, 2012 Service Based Award	Jun. 30, 2013 Service Based Award	Jun. 30, 2012 Service Based Award	Jun. 30, 2013 Deferred Stock Units	Jun. 30, 2012 Deferred Stock Units	Jun. 30, 2012 Unrestricted Shares	Jun. 30, 2013 Deferred stock units, performance-based stock units and restricted stock	Jun. 30, 2012 Deferred stock units, performance-based stock units and restricted stock	Jun. 30, 2013 Deferred stock units, performance-based stock units and restricted stock	Jun. 30, 2012 Deferred stock units, performance-based stock units and restricted stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options vested			0	0
Stock-based compensation expense	$ 0.1	$ 0.1	$ 0.1	$ 0.2	$ 0.1	$ 0.1	$ 0.2	$ 0.1	$ 0.1	$ 0.3	$ 0.2	$ 0.4			$ 0.1	$ 0.6	$ 0.3	$ 0.7	$ 0.5
Options issued			0	0			0	0			28,000	43,000
Stock-based compensation, maximum number of unvested shares			956,000
Expiration period			8 years
Performance based maximum number of unvested shares							805,000
Expiration term											8 years
Vesting period											4 years
Deferred stock units issued to non-employee directors													44,000	22,000
Vesting term													Deferred stock units issued to directors in lieu of directors fees are 100% vested at the grant date; matching deferred stock units equal to 25% of directors’ fees deferred vest one year following the grant date or, if earlier, upon reaching age 65.
Percentage of shares vested in lieu of directors fees at the grant date													100.00%
Portion of Director fees matched to deferred stock units													25.00%
Vesting period of deferred stock													1 year
Minimum age required for meeting directors fees deferred vest criteria													65 years
Share based compensation arrangement by share based payment award equity instruments other than options issued in period															5,000