Employee Benefit Plans - Component of Net Periodic Pension Cost (Benefit) (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components of net periodic cost:
Interest cost		$ 89	$ 91	$ 91
Amortization of prior service cost		0	0	9
Actuarial (gain) loss		87
Expected Amortization Of Pre Merger Regulatory Asset		0
Amortization of pre-merger regulatory asset		769
Net periodic postretirement cost		$ 176	$ 178	$ 199
Assumptions
Discount rate		3.75%	3.75%	3.50%
Defined Benefit Plan, Fair Value of Plan Assets		$ 57,746	$ 51,940
Chesapeake Postretirement Plan
Components of net periodic cost:
Interest cost		41	43	$ 42
Amortization of prior service cost		(77)	(77)	(77)
Actuarial (gain) loss		53	64	72
Net periodic pension cost		17	30	37
Expected Amortization Of Pre Merger Regulatory Asset			0	0
Amortization of pre-merger regulatory asset		0
Net periodic postretirement cost		$ 17	$ 30	$ 37
Assumptions
Discount rate		3.75%	3.75%	3.50%
Defined Benefit Plan, Benefit Obligation, Contributions by Plan Participant		$ 118	$ 90
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)		72	20
Defined Benefit Plan, Benefit Obligation, Benefits Paid		(235)	(174)
Defined Benefit Plan, Fair Value of Plan Assets		0	0	$ 0
Defined Benefit Plan, Plan Assets, Contributions by Employer	[1]	117	84
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant		118	90
Defined Benefit Plan, Plan Assets, Benefits Paid		(235)	(174)
Funded status		(1,128)	(1,132)
Defined Benefit Plan Accrued Pension Costs Accrued Expenses Employment Benefits		$ 1,128	$ 1,132
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate		3.50%	3.75%
FPU Medical Plan
Components of net periodic cost:
Interest cost		$ 50	$ 55	57
Amortization of prior service cost		0	0	0
Actuarial (gain) loss		0	0	0
Net periodic pension cost		50	55	57
Amortization of pre-merger regulatory asset		8	8	8
Net periodic postretirement cost		$ 58	$ 63	$ 65
Assumptions
Discount rate		4.00%	4.00%	3.75%
Defined Benefit Plan, Benefit Obligation, Contributions by Plan Participant		$ 48	$ 64
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)		(48)	(41)
Defined Benefit Plan, Benefit Obligation, Benefits Paid		(112)	(173)
Defined Benefit Plan, Fair Value of Plan Assets		0	0	$ 0
Defined Benefit Plan, Plan Assets, Contributions by Employer	[1]	64	109
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant		48	64
Defined Benefit Plan, Plan Assets, Benefits Paid		(112)	(173)
Funded status		(1,287)	(1,349)
Defined Benefit Plan Accrued Pension Costs Accrued Expenses Employment Benefits		$ 1,287	$ 1,349
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate		3.75%	4.00%
Chesapeake SERP
Components of net periodic cost:
Interest cost		$ 89	$ 91
Actuarial (gain) loss			87	99
Amortization of pre-merger regulatory asset		0
Assumptions
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)		63	(21)
Defined Benefit Plan, Benefit Obligation, Benefits Paid		(152)	(152)
Defined Benefit Plan, Plan Assets, Contributions by Employer		152	152
Defined Benefit Plan, Plan Assets, Benefits Paid		(152)	(152)
Funded status		(2,428)	(2,428)
Defined Benefit Plan Accrued Pension Costs Accrued Expenses Employment Benefits		$ 2,428	$ 2,428
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate		3.50%	3.75%
Chesapeake Pension Plan
Components of net periodic cost:
Interest cost		$ 402	$ 421	407
Expected return on assets		(495)	(501)	(530)
Actuarial (gain) loss		399	459	392
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement		0	161
Net periodic pension cost		306	540	269
Amortization of pre-merger regulatory asset		0	0	0
Net periodic postretirement cost		$ 306	$ 540	$ 269
Assumptions
Discount rate		3.75%	3.75%	3.50%
Expected return on plan assets		6.00%	6.00%	6.00%
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)		$ 454	$ 330
Defined Benefit Plan, Benefit Obligation, Benefits Paid		(768)	(464)
Defined Benefit Plan, Fair Value of Plan Assets		9,350	8,668	$ 8,752
Defined Benefit Plan, Plan Assets, Contributions by Employer		306	389
Defined Benefit Plan, Plan Assets, Benefits Paid		(768)	(464)
Funded status		(2,093)	(2,687)
Defined Benefit Plan Accrued Pension Costs Accrued Expenses Employment Benefits		$ 2,093	$ 2,687
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate		3.50%	3.75%
FPU Pension Plan
Components of net periodic cost:
Interest cost		$ 2,482	$ 2,525	2,504
Expected return on assets		(2,779)	(2,702)	(3,107)
Actuarial (gain) loss		513	519	456
Net periodic pension cost		216	342	(147)
Amortization of pre-merger regulatory asset		761	761	761
Net periodic postretirement cost		$ 977	$ 1,103	$ 614
Assumptions
Discount rate		4.00%	4.00%	3.75%
Expected return on plan assets		6.50%	6.50%	7.00%
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)		$ 1,199	$ (216)
Defined Benefit Plan, Benefit Obligation, Benefits Paid		(2,849)	(2,912)
Defined Benefit Plan, Fair Value of Plan Assets		48,396	43,272	$ 42,207
Defined Benefit Plan, Plan Assets, Contributions by Employer		1,948	1,634
Defined Benefit Plan, Plan Assets, Benefits Paid		(2,849)	(2,912)
Funded status		(16,268)	(20,560)
Defined Benefit Plan Accrued Pension Costs Accrued Expenses Employment Benefits		$ 16,268	$ 20,560
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, 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.