Employee Benefit Plans - Schedule of Funded Status of Benefit Obligation and Plan Assets (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in benefit obligation:
Interest cost		$ 89	$ 91	$ 91
Change in plan assets:
Balance, beginning of year		51,940
Balance, end of year		57,746	51,940
Chesapeake Pension Plan
Change in benefit obligation:
Benefit obligation - beginning of year		11,355	11,501
Interest cost		402	421	407
Actuarial loss		454	330
Effect of settlement		0	(433)
Benefits paid		(768)	(464)
Benefit obligation - end of year		11,443	11,355	11,501
Change in plan assets:
Balance, beginning of year		8,668	8,752
Actual return on plan assets		1,144	424
Employer contributions		306	389
Benefits paid		(768)	(464)
Effect of settlement		0	(433)
Balance, end of year		9,350	8,668	$ 8,752
Reconciliation:
Funded status		(2,093)	(2,687)
Accrued pension cost		$ (2,093)	$ (2,687)
Assumptions:
Discount rate		3.50%	3.75%
Expected return on plan assets		6.00%	6.00%	6.00%
FPU Pension Plan
Change in benefit obligation:
Benefit obligation - beginning of year		$ 63,832	$ 64,435
Interest cost		2,482	2,525	$ 2,504
Actuarial loss		1,199	(216)
Benefits paid		(2,849)	(2,912)
Benefit obligation - end of year		64,664	63,832	64,435
Change in plan assets:
Balance, beginning of year		43,272	42,207
Actual return on plan assets		6,025	2,343
Employer contributions		1,948	1,634
Benefits paid		(2,849)	(2,912)
Balance, end of year		48,396	43,272	$ 42,207
Reconciliation:
Funded status		(16,268)	(20,560)
Accrued pension cost		$ (16,268)	$ (20,560)
Assumptions:
Discount rate		3.75%	4.00%
Expected return on plan assets		6.50%	6.50%	7.00%
Chesapeake SERP
Change in benefit obligation:
Benefit obligation - beginning of year		$ 2,428	$ 2,510
Interest cost		89	91
Actuarial loss		63	(21)
Benefits paid		(152)	(152)
Benefit obligation - end of year		2,428	2,428	$ 2,510
Change in plan assets:
Employer contributions		152	152
Benefits paid		(152)	(152)
Reconciliation:
Funded status		(2,428)	(2,428)
Accrued pension cost		$ (2,428)	$ (2,428)
Assumptions:
Discount rate		3.50%	3.75%
Chesapeake Postretirement Plan
Change in benefit obligation:
Benefit obligation - beginning of year		$ 1,132	$ 1,153
Interest cost		41	43	42
Plan participants contributions		118	90
Actuarial loss		72	20
Benefits paid		(235)	(174)
Benefit obligation - end of year		1,128	1,132	1,153
Change in plan assets:
Balance, beginning of year		0	0
Employer contributions	[1]	117	84
Plan participants contributions		118	90
Benefits paid		(235)	(174)
Balance, end of year		0	0	0
Reconciliation:
Funded status		(1,128)	(1,132)
Accrued pension cost		$ (1,128)	$ (1,132)
Assumptions:
Discount rate		3.50%	3.75%
FPU Medical Plan
Change in benefit obligation:
Benefit obligation - beginning of year		$ 1,349	$ 1,444
Interest cost		50	55	57
Plan participants contributions		48	64
Actuarial loss		(48)	(41)
Benefits paid		(112)	(173)
Benefit obligation - end of year		1,287	1,349	1,444
Change in plan assets:
Balance, beginning of year		0	0
Employer contributions	[1]	64	109
Plan participants contributions		48	64
Benefits paid		(112)	(173)
Balance, end of year		0	0	$ 0
Reconciliation:
Funded status		(1,287)	(1,349)
Accrued pension cost		$ (1,287)	$ (1,349)
Assumptions:
Discount rate		3.75%	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.