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Employee Benefit Plans (Pension Benefit Plan) - Schedule of Cash Equivalents and Investments Held at Fair Value (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]      
Available-for-sale securities $ 118,038 $ 190,723  
Total 26,624 20,045 $ 19,213
Corporate Bonds [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Available-for-sale securities 32,543 66,412  
Pension Benefit Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Cash and cash equivalents 3,005 6  
Available-for-sale securities 23,619 20,039  
Total 26,624 20,045  
Pension Benefit Plan [Member] | Corporate Bonds [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Available-for-sale securities 14,349 12,546  
Pension Benefit Plan [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Cash and cash equivalents 3,005 6  
Available-for-sale securities 23,619 20,039  
Total 26,624 20,045  
Pension Benefit Plan [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Corporate Bonds [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Available-for-sale securities 14,349 12,546  
Pension Benefit Plan [Member] | Government Bonds [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Available-for-sale securities 2,305 2,037  
Pension Benefit Plan [Member] | Government Bonds [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Available-for-sale securities 2,305 2,037  
Pension Benefit Plan [Member] | Large Cap Blend [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Available-for-sale securities 5,758 4,462  
Pension Benefit Plan [Member] | Large Cap Blend [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Available-for-sale securities 5,758 4,462  
Pension Benefit Plan [Member] | Large Cap Value [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Available-for-sale securities 309 249  
Pension Benefit Plan [Member] | Large Cap Value [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Available-for-sale securities 309 249  
Pension Benefit Plan [Member] | Balanced Fund [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Available-for-sale securities 898 745  
Pension Benefit Plan [Member] | Balanced Fund [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Available-for-sale securities $ 898 $ 745