Comprehensive Income (Details) - USD ($)
$ in Thousands |
12 Months Ended |
Dec. 31, 2016 |
Dec. 31, 2015 |
Dec. 31, 2014 |
| Accumulated Other Comprehensive Income (Loss) [Line Items] |
|
|
|
| Balance |
$ 846,856
|
$ 858,026
|
$ 785,203
|
| Net current period change |
30,489
|
|
|
| Reclassification adjustments for gains and losses reclassified in income |
(13,717)
|
|
|
| Balance |
862,486
|
846,856
|
858,026
|
| Before-Tax Amount [Abstract] |
|
|
|
| Unrealized holding gains on securities arising during the period |
18,357
|
(25,765)
|
58,831
|
| Less reclassification adjustment for gains and losses realized in income |
(17,379)
|
(18,941)
|
(18,231)
|
| Net change in unrealized gain |
978
|
(44,706)
|
40,600
|
| Liability for pension benefits [Abstract] |
|
|
|
| Reclassification adjustment for amortization of net losses from past experience and prior service costs |
1,888
|
6,020
|
3,737
|
| Net change arising from assumptions and plan changes and experience |
25,783
|
19,940
|
(35,271)
|
| Net change in liability for pension benefits |
27,671
|
25,960
|
(31,534)
|
| Net current period change |
28,649
|
(18,746)
|
9,066
|
| Deferred Tax (Expense) Benefit [Abstract] |
|
|
|
| Unrealized holding gains on securities arising during the period |
(3,596)
|
3,153
|
(8,161)
|
| Less reclassification adjustment for gains and losses realized in income |
2,511
|
2,564
|
3,444
|
| Net change in unrealized gain |
(1,085)
|
5,717
|
(4,717)
|
| Liability for pension benefits [Abstract] |
|
|
|
| Reclassification adjustment for amortization of net losses from past experience and prior service costs |
(737)
|
(2,348)
|
(1,457)
|
| Net change arising from assumptions and plan changes and experience |
(10,055)
|
(7,776)
|
13,755
|
| Net change in liability for pension benefits |
(10,792)
|
(10,124)
|
12,298
|
| Net current period change |
(11,877)
|
(4,407)
|
7,581
|
| Net-of-Tax Amount [Abstract] |
|
|
|
| Unrealized holding gains on securities arising during the period |
14,761
|
(22,612)
|
50,670
|
| Less reclassification adjustment for gains and losses realized in income |
(14,868)
|
(16,377)
|
(14,787)
|
| Net change in unrealized gain |
(107)
|
(38,989)
|
35,883
|
| Liability for pension benefits [Abstract] |
|
|
|
| Reclassification adjustment for amortization of net losses from past experience and prior service costs |
1,151
|
3,672
|
2,280
|
| Net change arising from assumptions and plan changes and experience |
15,728
|
12,164
|
(21,516)
|
| Net change in liability for pension benefits |
16,879
|
15,836
|
(19,236)
|
| Net current period change |
16,772
|
(23,153)
|
$ 16,647
|
| Unrealized Gain on Securities [Member] |
|
|
|
| Accumulated Other Comprehensive Income (Loss) [Line Items] |
|
|
|
| Balance |
62,478
|
|
|
| Net current period change |
14,761
|
|
|
| Reclassification adjustments for gains and losses reclassified in income |
(14,868)
|
|
|
| Balance |
62,371
|
62,478
|
|
| Liability for Pension Benefits [Member] |
|
|
|
| Accumulated Other Comprehensive Income (Loss) [Line Items] |
|
|
|
| Balance |
(36,855)
|
|
|
| Net current period change |
15,728
|
|
|
| Reclassification adjustments for gains and losses reclassified in income |
1,151
|
|
|
| Balance |
(19,976)
|
(36,855)
|
|
| Accumulated Other Comprehensive Income [Member] |
|
|
|
| Accumulated Other Comprehensive Income (Loss) [Line Items] |
|
|
|
| Balance |
25,623
|
|
|
| Balance |
$ 42,395
|
$ 25,623
|
|