Postretirement Benefits - Pension Assets by Components at Fair Value (Detail) (Details) - Pension Benefits [Member] - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 421,758	$ 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	$ 194,932	$ 173,356
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	226,826	204,268
Financial [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 47,522	$ 43,451
% to Total	11.00%	12.00%
Financial [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 47,522	$ 43,451
Utilities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 39,258	$ 46,144
% to Total	9.00%	12.00%
Utilities [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 39,258	$ 46,144
Energy [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 23,632	$ 25,023
% to Total	6.00%	7.00%
Energy [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 23,632	$ 25,023
Other corporates [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 84,388	$ 65,888
% to Total	20.00%	17.00%
Other corporates [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 84,388	$ 65,888
Corporate bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 194,800	$ 180,506
% to Total	46.00%	48.00%
Corporate bonds [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 194,800	$ 180,506
Exchange traded fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 181,449	$ 164,351
% to Total	43.00%	43.00%
Exchange traded fund [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 181,449	$ 164,351
Other bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 242	$ 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	$ 242	$ 256
Other long-term investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 5,366	$ 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	$ 5,366	$ 2,304
Guaranteed annuity contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 26,418	$ 21,202
% to Total	7.00%	6.00%
Guaranteed annuity contract [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 26,418	$ 21,202
Short-term investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 9,304	$ 3,984
% to Total	2.00%	1.00%
Short-term investments [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 9,304	$ 3,984
Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 4,179	$ 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	$ 4,179	$ 5,021