Employee Benefit Plans - Component of Net Periodic Pension Cost (Benefit) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Components of net periodic cost:
Expected Amortization Of Pre Merger Regulatory Asset	$ 0
Amortization of pre-merger regulatory asset	769
Chesapeake Pension Plan
Components of net periodic cost:
Interest cost	421	$ 407	$ 425
Expected return on assets	(501)	(530)	(516)
Actuarial (gain) loss	459	392	176
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements	161
Net periodic pension cost	540	269	85
Amortization of pre-merger regulatory asset	0	0	0
Net periodic postretirement cost	$ 540	$ 269	$ 85
Assumptions
Discount rate	3.75%	3.50%	4.25%
Expected return on plan assets	6.00%	6.00%	6.00%
FPU Pension Plan
Components of net periodic cost:
Interest cost	$ 2,525	$ 2,504	$ 2,613
Expected return on assets	(2,702)	(3,107)	(3,089)
Actuarial (gain) loss	519	456	8
Net periodic pension cost	342	(147)	(468)
Amortization of pre-merger regulatory asset	761	761	761
Net periodic postretirement cost	$ 1,103	$ 614	$ 293
Assumptions
Discount rate	4.00%	3.75%	4.75%
Expected return on plan assets	6.50%	7.00%	7.00%
Chesapeake SERP
Components of net periodic cost:
Interest cost	$ 91	$ 91	$ 92
Amortization of prior service cost	0	9	19
Actuarial (gain) loss	87	99	47
Amortization of pre-merger regulatory asset	0
Net periodic postretirement cost	$ 178	$ 199	$ 158
Assumptions
Discount rate	3.75%	3.50%	4.25%
Chesapeake Postretirement Plan
Components of net periodic cost:
Interest cost	$ 43	$ 42	$ 39
Amortization of prior service cost	(77)	(77)	(77)
Actuarial (gain) loss	64	72	55
Net periodic pension cost	30	37	17
Expected Amortization Of Pre Merger Regulatory Asset		0	0
Amortization of pre-merger regulatory asset	0
Net periodic postretirement cost	$ 30	$ 37	$ 17
Assumptions
Discount rate	3.75%	3.50%	4.25%
FPU Medical Plan
Components of net periodic cost:
Interest cost	$ 55	$ 57	$ 69
Amortization of prior service cost	0	0	0
Actuarial (gain) loss	0	0	0
Net periodic pension cost	55	57	69
Amortization of pre-merger regulatory asset	8	8	8
Net periodic postretirement cost	$ 63	$ 65	$ 77
Assumptions
Discount rate	4.00%	3.75%	4.75%