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Net Defined Benefit Plan Costs (Detail) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Compensation And Retirement Disclosure [Abstract]          
Service costs $ 1,995 $ 1,720 $ 7,735 $ 5,661 $ 5,578
Interest costs 995 734 3,252 2,585 2,629
Amortization of actuarial loss 320 205 1,177 (113) 330
Expected return on plan assets (736) (492) (2,412) (2,043) (2,154)
One-time cost     209    
Special termination benefits     426    
Net defined benefit plan costs $ 2,574 $ 2,167 $ 10,387 $ 6,090 $ 6,383