Postretirement Benefits - Portion of Other Comprehensive Income Expected to Be Reflected in Pension Expense in Next Year (Detail) $ in Thousands	12 Months Ended
Dec. 31, 2016 USD ($)
Postretirement Benefits [Abstract]
Amortization of prior service cost	$ 476
Amortization of net actuarial loss	11,806
Total	$ 12,282

Amount included in accumulated other comprehensive income (AOCI) expected to be recognized as components of net periodic benefit cost for the fiscal year that follows the most recent annual statement of financial position, including gain (loss), prior service cost (credit), and transition asset (obligation).

No definition available.

Name:	us-gaap_DefinedBenefitPlanAmountToBeAmortizedFromAccumulatedOtherComprehensiveIncomeLossNextFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration