Postretirement Benefit Plans - Pension Assets by Components at Fair Value (Detail) (Details) - Pension Benefits [Member] - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 365,774	$ 377,624
% to Total	100.00%	100.00%
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 168,673	$ 173,356
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	197,101	204,268
Corporate bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 173,091	$ 180,506
% to Total	47.00%	48.00%
Corporate bonds [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 173,091	$ 180,506
Corporate bonds [Member] | Financial [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 41,709	$ 43,451
% to Total	11.00%	12.00%
Corporate bonds [Member] | Financial [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 41,709	$ 43,451
Corporate bonds [Member] | Utilities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 44,870	$ 46,144
% to Total	12.00%	12.00%
Corporate bonds [Member] | Utilities [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 44,870	$ 46,144
Corporate bonds [Member] | Energy [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 23,846	$ 25,023
% to Total	7.00%	7.00%
Corporate bonds [Member] | Energy [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 23,846	$ 25,023
Corporate bonds [Member] | Other corporate sectors [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 62,666	$ 65,888
% to Total	17.00%	17.00%
Corporate bonds [Member] | Other corporate sectors [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 62,666	$ 65,888
Exchange traded fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 162,980	$ 164,351
% to Total	45.00%	43.00%
Exchange traded fund [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 162,980	$ 164,351
Other bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 249	$ 256
% to Total	0.00%	0.00%
Other bonds [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 249	$ 256
Other long-term investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 2,439	$ 2,304
% to Total	1.00%	1.00%
Other long-term investments [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 2,439	$ 2,304
Guaranteed annuity contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 21,322	$ 21,202
% to Total	6.00%	6.00%
Guaranteed annuity contract [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 21,322	$ 21,202
Short-term investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 2,070	$ 3,984
% to Total	0.00%	1.00%
Short-term investments [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 2,070	$ 3,984
Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 3,623	$ 5,021
% to Total	1.00%	1.00%
Other [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 3,623	$ 5,021