Pension and Other Employee Benefits - Schedule of Asset Allocations for Funded Retirement Plan (Detail)	Dec. 31, 2017	Dec. 31, 2016
U.S. Government Bonds [Member]
Defined Benefit Pension Plan With Accumulated Benefit Obligation In Excess Of Fair Value Of Plan Assets [Line Items]
Actual	16.00%	15.00%
U.S. Government Bonds [Member] | Minimum [Member]
Defined Benefit Pension Plan With Accumulated Benefit Obligation In Excess Of Fair Value Of Plan Assets [Line Items]
Target, Minimum	0.00%	0.00%
U.S. Government Bonds [Member] | Maximum [Member]
Defined Benefit Pension Plan With Accumulated Benefit Obligation In Excess Of Fair Value Of Plan Assets [Line Items]
Target, Minimum		50.00%
Investment Grade Bonds [Member]
Defined Benefit Pension Plan With Accumulated Benefit Obligation In Excess Of Fair Value Of Plan Assets [Line Items]
Actual	71.00%	72.00%
Investment Grade Bonds [Member] | Minimum [Member]
Defined Benefit Pension Plan With Accumulated Benefit Obligation In Excess Of Fair Value Of Plan Assets [Line Items]
Target, Minimum	0.00%	0.00%
Investment Grade Bonds [Member] | Maximum [Member]
Defined Benefit Pension Plan With Accumulated Benefit Obligation In Excess Of Fair Value Of Plan Assets [Line Items]
Target, Minimum		100.00%
High Yield Bonds [Member]
Defined Benefit Pension Plan With Accumulated Benefit Obligation In Excess Of Fair Value Of Plan Assets [Line Items]
Actual	12.00%	12.00%
High Yield Bonds [Member] | Minimum [Member]
Defined Benefit Pension Plan With Accumulated Benefit Obligation In Excess Of Fair Value Of Plan Assets [Line Items]
Target, Minimum	0.00%	0.00%
High Yield Bonds [Member] | Maximum [Member]
Defined Benefit Pension Plan With Accumulated Benefit Obligation In Excess Of Fair Value Of Plan Assets [Line Items]
Target, Minimum		100.00%
Cash [Member]
Defined Benefit Pension Plan With Accumulated Benefit Obligation In Excess Of Fair Value Of Plan Assets [Line Items]
Actual	1.00%	1.00%
Cash [Member] | Minimum [Member]
Defined Benefit Pension Plan With Accumulated Benefit Obligation In Excess Of Fair Value Of Plan Assets [Line Items]
Target, Minimum	0.00%	0.00%
Cash [Member] | Maximum [Member]
Defined Benefit Pension Plan With Accumulated Benefit Obligation In Excess Of Fair Value Of Plan Assets [Line Items]
Target, Minimum		5.00%