Employee Benefit Plans - Schedule of Assets by Investment Type (Detail)	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Chesapeake Pension Plan
Asset Category
Percentage of assets by investment type	100.00%	100.00%	100.00%
Chesapeake Pension Plan | Investments in equity securities
Asset Category
Percentage of assets by investment type	52.70%	52.93%	48.01%
Chesapeake Pension Plan | Debt securities
Asset Category
Percentage of assets by investment type	37.79%	37.64%	39.62%
Chesapeake Pension Plan | Other
Asset Category
Percentage of assets by investment type	9.51%	9.43%	12.37%
FPU Pension Plan
Asset Category
Percentage of assets by investment type	100.00%	100.00%	100.00%
FPU Pension Plan | Investments in equity securities
Asset Category
Percentage of assets by investment type	55.17%	53.18%	48.56%
FPU Pension Plan | Debt securities
Asset Category
Percentage of assets by investment type	36.56%	37.74%	41.74%
FPU Pension Plan | Other
Asset Category
Percentage of assets by investment type	8.27%	9.08%	9.70%