Benefit Plans - Change in Projected Benefit Obligation (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Change in projected benefit obligation:
Projected benefit obligation at beginning of year	$ 145,034		$ 161,065
Service cost	0		7,104		7,338
Interest cost	6,897		6,645		6,516
Plan amendments	0		119
Actuarial loss (gain)	28,749		(13,003)
Curtailments	0		(12,981)
Benefits paid	(4,341)		(3,915)
Projected benefit obligation at end of year	176,339		145,034		161,065
Change in fair value of plan assets:
Fair value of plan assets at beginning of year	184,910	[1]	138,857
Actual return on plan assets	13,813		24,723
Employer contributions	264		25,245
Benefits paid	(4,341)		(3,915)
Fair value of plan assets at end of year	194,646	[1]	184,910	[1]	138,857
Funded status of the plan	18,307		39,876
Asset recognized	18,307		39,876
Accumulated benefit obligation	$ 176,339		$ 145,034

[1] Includes accrued interest receivable of $652 thousand and $704 thousand as of December 31, 2014 and 2013, respectively.