Employer Sponsored Benefit Plan - Trust Assets at Fair Value (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	$ 44,174	$ 37,038
Measured at NAV	20,414	14,362
Domestic equities
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	11,577	10,964
Measured at NAV	0	0
International equities
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	11,626	10,944
Measured at NAV	0	0
Fixed income
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	17,360	10,499
Measured at NAV	17,360	10,499
Hedge fund of funds
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	3,054	3,863
Measured at NAV	3,054	3,863
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	557	768
Measured at NAV	0	0
Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	23,760	22,676
Level 1 | Domestic equities
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	11,577	10,964
Level 1 | International equities
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	11,626	10,944
Level 1 | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Level 1 | Hedge fund of funds
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Level 1 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	557	768
Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Level 2 | Domestic equities
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Level 2 | International equities
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Level 2 | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Level 2 | Hedge fund of funds
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Level 2 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Level 3
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Level 3 | Domestic equities
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Level 3 | International equities
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Level 3 | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Level 3 | Hedge fund of funds
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Level 3 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	$ 0	$ 0