Pensions - Pension Plan Assets at Fair Value (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value Of Pension Plan Assets [Line Items]
Pension plan assets at fair value	$ 327,793	$ 316,703
Equity securities [Member]
Fair Value Of Pension Plan Assets [Line Items]
Pension plan assets at fair value	63,499	55,241
Debt securities [Member]
Fair Value Of Pension Plan Assets [Line Items]
Pension plan assets at fair value	64,604	63,409
Balanced funds [Member]
Fair Value Of Pension Plan Assets [Line Items]
Pension plan assets at fair value	3,713	680
Insurance Contract, Rights and Obligations [Member]
Fair Value Of Pension Plan Assets [Line Items]
Pension plan assets at fair value	0	16,113
Cash and cash equivalents [Member]
Fair Value Of Pension Plan Assets [Line Items]
Pension plan assets at fair value	4,898	12,028
Net Asset Value [Member] | Equity securities [Member]
Fair Value Of Pension Plan Assets [Line Items]
Pension plan assets at fair value	76,405	76,961
Net Asset Value [Member] | Debt securities [Member]
Fair Value Of Pension Plan Assets [Line Items]
Pension plan assets at fair value	69,823	47,123
Net Asset Value [Member] | Real Estate [Member]
Fair Value Of Pension Plan Assets [Line Items]
Pension plan assets at fair value	15,656	14,472
Net Asset Value [Member] | Balanced funds [Member]
Fair Value Of Pension Plan Assets [Line Items]
Pension plan assets at fair value	29,195	30,676
Level 1 [Member]
Fair Value Of Pension Plan Assets [Line Items]
Pension plan assets at fair value	84,578	84,416
Level 1 [Member] | Equity securities [Member]
Fair Value Of Pension Plan Assets [Line Items]
Pension plan assets at fair value	41,080	36,710
Level 1 [Member] | Debt securities [Member]
Fair Value Of Pension Plan Assets [Line Items]
Pension plan assets at fair value	34,997	35,339
Level 1 [Member] | Balanced funds [Member]
Fair Value Of Pension Plan Assets [Line Items]
Pension plan assets at fair value	3,603	339
Level 1 [Member] | Insurance Contract, Rights and Obligations [Member]
Fair Value Of Pension Plan Assets [Line Items]
Pension plan assets at fair value	0	0
Level 1 [Member] | Cash and cash equivalents [Member]
Fair Value Of Pension Plan Assets [Line Items]
Pension plan assets at fair value	4,898	12,028
Level 2 [Member]
Fair Value Of Pension Plan Assets [Line Items]
Pension plan assets at fair value	52,026	46,601
Level 2 [Member] | Equity securities [Member]
Fair Value Of Pension Plan Assets [Line Items]
Pension plan assets at fair value	22,419	18,531
Level 2 [Member] | Debt securities [Member]
Fair Value Of Pension Plan Assets [Line Items]
Pension plan assets at fair value	29,607	28,070
Level 2 [Member] | Balanced funds [Member]
Fair Value Of Pension Plan Assets [Line Items]
Pension plan assets at fair value	0	0
Level 2 [Member] | Insurance Contract, Rights and Obligations [Member]
Fair Value Of Pension Plan Assets [Line Items]
Pension plan assets at fair value	0	0
Level 2 [Member] | Cash and cash equivalents [Member]
Fair Value Of Pension Plan Assets [Line Items]
Pension plan assets at fair value	0	0
Level 3 [Member]
Fair Value Of Pension Plan Assets [Line Items]
Pension plan assets at fair value	110	16,454
Level 3 [Member] | Equity securities [Member]
Fair Value Of Pension Plan Assets [Line Items]
Pension plan assets at fair value	0	0
Level 3 [Member] | Debt securities [Member]
Fair Value Of Pension Plan Assets [Line Items]
Pension plan assets at fair value	0	0
Level 3 [Member] | Balanced funds [Member]
Fair Value Of Pension Plan Assets [Line Items]
Pension plan assets at fair value	110	341
Level 3 [Member] | Insurance Contract, Rights and Obligations [Member]
Fair Value Of Pension Plan Assets [Line Items]
Pension plan assets at fair value	0	16,113
Level 3 [Member] | Cash and cash equivalents [Member]
Fair Value Of Pension Plan Assets [Line Items]
Pension plan assets at fair value	$ 0	$ 0