Fair value measurement - Fair Value of Pension Plan Assets by Level within Fair Value Hierarchy (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fair value of plan assets	$ 439,149
Retirement Plans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fair value of plan assets	439,149	$ 388,168	$ 400,193
Total	128,819	69,739
Retirement Plans | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	128,819	69,739
Retirement Plans | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	0	0
Retirement Plans | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	0	0
Retirement Plans | Cash and other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fair value of plan assets	935	2,206
Retirement Plans | Cash and other | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fair value of plan assets	935	2,206
Retirement Plans | Cash and other | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fair value of plan assets	0	0
Retirement Plans | Cash and other | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fair value of plan assets	0	0
Retirement Plans | Corporate stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fair value of plan assets	82,698	60,730
Retirement Plans | Corporate stock | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fair value of plan assets	82,698	60,730
Retirement Plans | Corporate stock | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fair value of plan assets	0	0
Retirement Plans | Corporate stock | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fair value of plan assets	0	0
Retirement Plans | Interest in registered investment companies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fair value of plan assets	45,186	6,803
Retirement Plans | Interest in registered investment companies | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fair value of plan assets	45,186	6,803
Retirement Plans | Interest in registered investment companies | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fair value of plan assets	0	0
Retirement Plans | Interest in registered investment companies | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fair value of plan assets	0	0
Retirement Plans | Common collective trust - equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Pension plan investments valued using net asset value as a practical expedient:	117,778	167,647
Retirement Plans | Common collective trust - fixed income
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Pension plan investments valued using net asset value as a practical expedient:	170,977	127,043
Retirement Plans | Hedge funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Pension plan investments valued using net asset value as a practical expedient:	15,756	14,754
Retirement Plans | Partnership/joint venture interests
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Pension plan investments valued using net asset value as a practical expedient:	$ 5,819	$ 8,985