Pension and Post-Retirement Benefits - Changes in Benefit Obligations and Plan Assets (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Pension Plans, Postretirement and Other Employee Benefits
Fair value of plan assets, nonqualified pension plan		$ 0	$ 0
Projected benefit obligation, nonqualified pension plan		1,600	1,700
Defined Benefit Plan, Pension Plans with Accumulated Benefit Obligations in Excess of Plan Assets, Aggregate Accumulated Benefit Obligation		1,600	1,700
Pension Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning balance		122,336	99,828
Service cost	$ 1,280	1,295	1,080
Interest cost	4,198	4,813	4,811
Actuarial (gain) loss		(7,492)	22,163
Benefits paid		(6,106)	(6,017)
Amendments		574	471
Other		0	0
Benefit obligation, ending balance		115,420	122,336
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning balance		90,897	85,367
Actual return on plan assets		(2,100)	6,937
Employer contributions		2,025	4,610
Benefits paid		(6,106)	(6,017)
Other		0	0
Fair value of plan assets, ending balance		84,716	90,897
Plan assets less than benefit obligations at December 31 recognized as liability for pension and other post-retirement benefits		(30,704)	(31,439)
Post-retirement Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning balance		28,289	22,261
Service cost	193	176	151
Interest cost	$ 947	1,074	1,030
Actuarial (gain) loss		(3,631)	5,340
Benefits paid		(1,288)	(996)
Amendments		0	0
Other		471	503
Benefit obligation, ending balance		25,091	28,289
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning balance		19	51
Actual return on plan assets		(2)	3
Employer contributions		864	492
Benefits paid		(1,288)	(996)
Other		424	469
Fair value of plan assets, ending balance		17	19
Plan assets less than benefit obligations at December 31 recognized as liability for pension and other post-retirement benefits		$ (25,074)	$ (28,270)