Employee Benefit Plans - Obligation and Funded Status of Defined Benefit Pension Plan and Retirement Plan (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Change in benefit obligation
Service cost	$ 693		$ 843		$ 749
Interest cost	1,278		1,160		1,167
Change in plan assets
Fair value of plan assets at beginning of year	18,367
Fair value of plan assets at end of year	19,726		18,367
Unrecognized loss included in accumulated other comprehensive loss
Amortization	419		1,093		1,147
Defined Benefit Pension and Other Postretirement Plans
Change in benefit obligation
Benefit obligation at beginning of year	28,438	[1]	31,142	[1]
Service cost	693	[1]	843	[1]
Interest cost	1,278	[1]	1,160	[1]
Actuarial (gain) loss	4,117	[1]	(3,372)	[1]
Benefits paid	(1,227)	[1]	(1,335)	[1]
Ending benefit obligation (December 31)	33,299		28,438	[1]
Change in plan assets
Fair value of plan assets at beginning of year	18,367		14,687
Return on assets	669		2,509
Employer contribution	1,916		2,506
Benefits paid	(1,227)	[1]	(1,335)	[1]
Fair value of plan assets at end of year	19,725		18,367
Funded status (under funded)	(13,574)		(10,071)
Amounts recognized in the consolidated balance sheet
Current liabilities	464		497
Non-current liabilities	13,110		9,574
Unrecognized loss included in accumulated other comprehensive loss
Unrecognized loss before tax, beginning balance	6,183		11,934
Amortization	(419)		(1,093)
Liability (gain) loss	4,117		(3,372)
Asset (gain) loss	733		(1,286)
Unrecognized loss before tax, ending balance	10,614		6,183
Tax impact	(3,970)		(2,321)
Unrecognized loss included in accumulated other comprehensive loss, net of tax	6,644		3,862
Prepaid benefit cost included in other assets	1,832		1,135
Accrued benefit cost included in other liabilities	$ 4,795		$ 5,024

[1] Projected benefit obligation equals the accumulated benefit obligation for the plans.