Employee Benefit Plans - Amounts in Accumulated Other Comprehensive Income/Loss and Regulatory Asset (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Prior service cost (credit)	$ (77)
Net loss	944
Amortization of pre-merger regulatory asset	769
Chesapeake Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Prior service cost (credit)	0
Net loss	351
Amortization of pre-merger regulatory asset	0	$ 0	$ 0
FPU Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Prior service cost (credit)	0
Net loss	434
Amortization of pre-merger regulatory asset	761	761	761
Chesapeake SERP
Defined Benefit Plan Disclosure [Line Items]
Prior service cost (credit)	0
Net loss	101
Amortization of pre-merger regulatory asset	0
Chesapeake Postretirement Plan
Defined Benefit Plan Disclosure [Line Items]
Prior service cost (credit)	(77)
Net loss	58
Amortization of pre-merger regulatory asset	0
FPU Medical Plan
Defined Benefit Plan Disclosure [Line Items]
Prior service cost (credit)	0
Net loss	0
Amortization of pre-merger regulatory asset	$ 8	$ 8	$ 8