Retirement Plans - Schedule of Projected Benefit Obligation, Accumulated Benefit Obligation and Fair Value of Plan Assets for Qualified and Non-Qualified Pension Plans (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 156,924	$ 139,731	$ 153,958
Accumulated benefit obligation	156,589	139,483
Fair value of plan assets	140,140	139,737	113,416
Qualified Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	135,998	120,512
Accumulated benefit obligation	135,998	120,512
Fair value of plan assets	140,140	139,737
Non-Qualified Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	20,926	19,219
Accumulated benefit obligation	$ 20,591	$ 18,971