Postretirement Benefit Plans - Company's Asset Allocations for its U.S. Pension Plans by Asset Category (Detail) - United States [Member] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Total	$ 147,908	$ 137,092	$ 121,835
Commodities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total		2,523
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	109,979	99,872
Level 1 [Member] | Commodities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total		2,523
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	37,929	37,220
Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	4,903	3,908
Cash and Cash Equivalents [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	4,903	3,633
Cash and Cash Equivalents [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total		275
U.S. Equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	37,753	29,390
U.S. Equities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	37,753	29,390
Non-U.S. Equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	31,628	14,637
Non-U.S. Equities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	31,581	14,637
Non-U.S. Equities [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	47
Employer Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	30,197	36,018
Employer Securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	30,197	36,018
Equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	99,578	80,045
Equities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	99,531	80,045
Equities [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	47
U.S. Corporate Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	28,744	28,278
U.S. Corporate Bonds [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	28,744	28,278
U.S. Government and Agency Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	6,590	9,280
U.S. Government and Agency Bonds [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	5,545	8,309
U.S. Government and Agency Bonds [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	1,045	971
Other Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	8,093	7,696
Other Bonds [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	8,093	7,696
Debt Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	43,427	45,254
Debt Securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	5,545	8,309
Debt Securities [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total	$ 37,882	36,945
Real Estate Fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Total		5,362
Real Estate Fund [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total		5,362
Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Total		7,885
Mutual Funds | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total		$ 7,885