Components of Retirement Plan Costs (Detail) - USD ($)
$ in Thousands |
3 Months Ended |
6 Months Ended |
Jun. 30, 2016 |
Jun. 30, 2015 |
Jun. 30, 2016 |
Jun. 30, 2015 |
| Pension Plans |
|
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items] |
|
|
|
|
| Service Cost |
$ 851
|
$ 916
|
$ 1,702
|
$ 1,832
|
| Interest Cost |
1,486
|
1,343
|
2,972
|
2,686
|
| Expected Return on Plan Assets |
(1,814)
|
(1,694)
|
(3,628)
|
(3,388)
|
| Prior Service Cost Amortization |
66
|
54
|
132
|
108
|
| Actuarial Loss Amortization |
1,099
|
1,180
|
2,198
|
2,360
|
| Sub-total |
1,688
|
1,799
|
3,376
|
3,598
|
| Amounts Capitalized and Deferred |
(797)
|
(875)
|
(1,407)
|
(1,583)
|
| Net Periodic Benefit Cost Recognized |
891
|
924
|
1,969
|
2,015
|
| Other Postretirement Benefit Plans, Defined Benefit |
|
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items] |
|
|
|
|
| Service Cost |
652
|
656
|
1,304
|
1,312
|
| Interest Cost |
808
|
729
|
1,616
|
1,458
|
| Expected Return on Plan Assets |
(301)
|
(273)
|
(602)
|
(546)
|
| Prior Service Cost Amortization |
372
|
420
|
744
|
840
|
| Actuarial Loss Amortization |
262
|
288
|
524
|
576
|
| Sub-total |
1,793
|
1,820
|
3,586
|
3,640
|
| Amounts Capitalized and Deferred |
(867)
|
(881)
|
(1,604)
|
(1,642)
|
| Net Periodic Benefit Cost Recognized |
926
|
939
|
1,982
|
1,998
|
| Supplemental Employee Retirement Plans, Defined Benefit |
|
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items] |
|
|
|
|
| Service Cost |
41
|
28
|
82
|
56
|
| Interest Cost |
96
|
78
|
192
|
156
|
| Prior Service Cost Amortization |
47
|
3
|
94
|
6
|
| Actuarial Loss Amortization |
94
|
84
|
188
|
168
|
| Sub-total |
278
|
193
|
556
|
386
|
| Net Periodic Benefit Cost Recognized |
$ 278
|
$ 193
|
$ 556
|
$ 386
|