Benefit Plans - Change in Projected Benefit Obligation (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Change in projected benefit obligation:
Projected benefit obligation at beginning of year	$ 133,796		$ 108,146
Service cost	7,253		5,951		5,544
Interest cost	6,429		6,147		5,709
Plan amendments	556		338
Actuarial loss	14,040		16,678
Benefits paid	(3,510)		(3,464)
Projected benefit obligation at end of year	158,564		133,796		108,146
Change in fair value of plan assets:
Fair value of plan assets at beginning of year	95,746	[1]	83,937
Actual return on plan assets	11,516		2,168
Employer contributions	35,105		13,105
Benefits paid	(3,510)		(3,464)
Fair value of plan assets at end of year	138,857	[1]	95,746	[1]	83,937
Funded status of the plan	(19,707)		(38,050)
Liability recognized	(19,707)		(38,050)
Accumulated benefit obligation	$ 141,889		$ 120,564

[1] Includes accrued interest receivable of $526 thousand and $309 thousand as of December 31, 2012 and 2011, respectively.