Retirement Plan, Based on Measurement Date (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Change in Plan Assets:
Fair value of assets at end of year	$ 211,888
Pension Benefits
Change in Benefit Obligation:
Benefit obligation at beginning of year	157,061	$ 126,841
Interest cost	6,063	5,895	$ 5,455
Actuarial (gain) loss	(7,891)	31,544
Annuity payments	(6,339)	(5,827)
Settlements	(2,276)	(1,392)
Benefit obligation at end of year	146,618	157,061	126,841
Change in Plan Assets:
Fair value of assets at beginning of year	222,990	219,330
Actual (loss) return on plan assets	(2,487)	10,879
Contributions	0	0
Annuity payments	(6,339)	(5,827)
Settlements	(2,276)	(1,392)
Fair value of assets at end of year	211,888	222,990	$ 219,330
Funded status (included in "Other assets")	65,270	65,929
Changes recognized in other comprehensive (loss) income for the year ended December 31:
Amortization of prior service cost	0	0
Amortization of actuarial loss	(8,208)	(3,289)
Net actuarial loss arising during the year	12,155	40,100
Total recognized in other comprehensive loss for the year (pre-tax)	3,947	36,811
Accumulated other comprehensive loss (pre-tax) not yet recognized in net periodic benefit cost at December 31:
Prior service cost	0	0
Actuarial loss, net	87,885	83,938
Total accumulated other comprehensive loss (pre-tax)	$ 87,885	$ 83,938