Summary of Stock Option Activity (Detail) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014
Shares
Options outstanding, beginning of year	533,600
Granted	0
Exercised	(232,600)
Cancelled	0
Options outstanding, end of year	301,000	[1]
Weighted Average Exercise Price
Options outstanding, beginning of year	$ 9.65
Granted	$ 0
Exercised	$ 9.83
Cancelled	$ 0
Options outstanding, end of year	$ 9.50	[1]
Weighted average remaining life
Options outstanding, end of year	1 year 4 months 24 days	[1]
Total Intrinsic value
Options outstanding, end of year	$ 11.6	[1]

[1] All options outstanding are exercisable