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Pension Benefits - Contributions and Expected Benefit Payments (Details) - Pension Plan [Member]
$ in Thousands
12 Months Ended
Dec. 31, 2017
USD ($)
Pension Benefits  
Future employer contributions in next fiscal year $ 8,000
2018 101,397
2019 102,884
2020 103,956
2021 105,752
2022 107,581
2023-2027 558,298
Qualified Plan [Member]  
Pension Benefits  
Pension contributions 127,600
2018 84,216
2019 85,816
2020 87,162
2021 89,169
2022 91,192
2023-2027 479,738
Nonqualified Plan [Member]  
Pension Benefits  
2018 17,181
2019 17,068
2020 16,794
2021 16,583
2022 16,389
2023-2027 $ 78,560