Employee Benefits - Summary of Defined Pension Plan and Supplemental Insurance Retirement Plan (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Net Periodic Benefit Cost [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	$ (465)	$ (475)
Net actuarial loss	(25,122)	(11,262)
Total	(25,587)	(11,737)
Defined Benefit Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	(104)	(104)
Net actuarial loss	8,398	(3,734)
Total	40,011	29,879	31,910
Defined Benefit Pension Plan [Member] | Net Periodic Benefit Cost [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	412	516
Net actuarial loss	(12,953)	(3,361)
Total	(12,541)	(2,845)
Supplemental Insurance/ Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	114	114
Net actuarial loss	4,650	(1,916)
Total	31,989	25,502	25,835
Supplemental Insurance/ Retirement Plan [Member] | Net Periodic Benefit Cost [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	(877)	(991)
Net actuarial loss	(12,169)	(7,901)
Total	$ (13,046)	$ (8,892)