Employee Benefit Plans - Schedule of Funded Status of Benefit Obligation and Plan Assets (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Change in plan assets:
Balance, beginning of year		$ 50,959
Balance, end of year		51,940	$ 50,959
Chesapeake Pension Plan
Change in benefit obligation:
Benefit obligation - beginning of year		11,501	11,981
Interest cost		421	407	$ 425
Actuarial loss		330	(401)
Effect of settlement		(433)
Benefits paid		(464)	(486)
Benefit obligation - end of year		11,355	11,501	11,981
Change in plan assets:
Balance, beginning of year		8,752	9,078
Actual return on plan assets		424	(289)
Employer contributions		389	449
Benefits paid		(464)	(486)
Effect of settlement		(433)
Balance, end of year		8,668	8,752	9,078
Reconciliation:
Funded status		(2,687)	(2,749)
Accrued pension cost		$ (2,687)	$ (2,749)
Assumptions:
Discount rate		3.75%	3.75%
FPU Pension Plan
Change in benefit obligation:
Benefit obligation - beginning of year		$ 64,435	$ 68,173
Interest cost		2,525	2,504	2,613
Actuarial loss		(216)	(3,374)
Benefits paid		(2,912)	(2,868)
Benefit obligation - end of year		63,832	64,435	68,173
Change in plan assets:
Balance, beginning of year		42,207	45,077
Actual return on plan assets		2,343	(1,464)
Employer contributions		1,634	1,462
Benefits paid		(2,912)	(2,868)
Balance, end of year		43,272	42,207	45,077
Reconciliation:
Funded status		(20,560)	(22,228)
Accrued pension cost		$ (20,560)	$ (22,228)
Assumptions:
Discount rate		4.00%	4.00%
Chesapeake SERP
Change in benefit obligation:
Benefit obligation - beginning of year		$ 2,510	$ 2,650
Interest cost		91	91	92
Actuarial loss		(21)	(85)
Benefits paid		(152)	(146)
Benefit obligation - end of year		2,428	2,510	2,650
Change in plan assets:
Employer contributions		152	146
Benefits paid		(152)	(146)
Reconciliation:
Funded status		(2,428)	(2,510)
Accrued pension cost		$ (2,428)	$ (2,510)
Assumptions:
Discount rate		3.75%	3.75%
Chesapeake Postretirement Plan
Change in benefit obligation:
Benefit obligation - beginning of year		$ 1,153	$ 1,238
Interest cost		43	42	39
Plan participants contributions		90	108
Actuarial loss		20	(58)
Benefits paid		(174)	(177)
Benefit obligation - end of year		1,132	1,153	1,238
Change in plan assets:
Balance, beginning of year		0	0
Employer contributions	[1]	84	69
Plan participants contributions		90	108
Benefits paid		(174)	(177)
Balance, end of year		0	0	0
Reconciliation:
Funded status		(1,132)	(1,153)
Accrued pension cost		$ (1,132)	$ (1,153)
Assumptions:
Discount rate		3.75%	3.75%
FPU Medical Plan
Change in benefit obligation:
Benefit obligation - beginning of year		$ 1,444	$ 1,712
Interest cost		55	57	69
Plan participants contributions		64	75
Actuarial loss		(41)	(132)
Benefits paid		(173)	(268)
Benefit obligation - end of year		1,349	1,444	1,712
Change in plan assets:
Balance, beginning of year		0	0
Employer contributions	[1]	109	193
Plan participants contributions		64	75
Benefits paid		(173)	(268)
Balance, end of year		0	0	$ 0
Reconciliation:
Funded status		(1,349)	(1,444)
Accrued pension cost		$ (1,349)	$ (1,444)
Assumptions:
Discount rate		4.00%	4.00%

[1] The Chesapeake Postretirement Plan does not receive a Medicare Part-D subsidy. The FPU Medical Plan did not receive a significant subsidy for the post-merger period.