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Postretirement Benefit Plans - Summary of Post-Retirement Benefit Costs by Component (Detail) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Pension Benefits [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 3,990 $ 3,231 $ 11,971 $ 9,694
Interest cost 5,003 4,695 15,009 14,333
Expected return on assets (5,323) (4,768) (15,969) (14,273)
Amortization:        
Prior service cost 81 529 244 1,586
Actuarial (gain) loss 3,533 2,035 10,601 6,127
Actuarial valuation adjustment 0 0 0 0
Direct recognition of expense 0 131 0 131
Net periodic benefit cost 7,284 5,853 21,856 17,598
Other Benefits [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 0 25 0 74
Interest cost 203 (63) 610 234
Expected return on assets 0 0 0 0
Amortization:        
Prior service cost 0 0 0 0
Actuarial (gain) loss 30 3 90 3
Actuarial valuation adjustment 0 460 0 460
Direct recognition of expense 166 114 493 10
Net periodic benefit cost $ 399 $ 539 $ 1,193 $ 781