Employer Contributions, Participant Contributions and Benefit Payments (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Plans
Defined Contribution Plan Disclosure [Line Items]
Employer Contributions	$ 5,146	$ 4,215	$ 4,191
Benefit Payments	4,900	4,410	4,246
Other Postretirement Benefit Plans, Defined Benefit
Defined Contribution Plan Disclosure [Line Items]
Employer Contributions	4,000	4,000	3,650
Participant Contributions	61	63	59
Benefit Payments	2,421	2,515	2,184
Supplemental Employee Retirement Plans, Defined Benefit
Defined Contribution Plan Disclosure [Line Items]
Employer Contributions	34	40	53
Benefit Payments	$ 34	$ 40	$ 53