Stock-based Compensation - Performance-based Stock Options and RSUs - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2013 Performance-based RSU [Member]	Dec. 31, 2012 Performance-based RSU [Member]	Dec. 31, 2011 Performance-based RSU [Member]	Dec. 31, 2012 Performance-Based RSUs Granted In 2012 [Member]	Dec. 31, 2011 Performance-based RSUs [Member]	Dec. 31, 2009 Performance-based stock options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
RSUs granted during the period		244,112	282,850	323,600	191,594	195,156
Estimated grant date fair value of restricted stock		$ 5.7	$ 4.9	$ 5.0	$ 3.0	$ 3.0
Service period, in years		1 year	1 year	1 year
RSUs that remain outstanding		240,374	0	0	191,594	124,390
Approximate percentage of performance criteria achieved			58.00%	90.00%
Average derived service period, in years					3 years	3 years	3 years 8 months 16 days
RSUs vested						64,909
Options granted during the period							32,674
Estimated grant date fair value of stock options							$ 0.1
Stock options vested							5,298
Stock options that remain outstanding							15,540