Employee Benefit Plans (Summary of Estimated Future Benefit Payments) (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
2018	$ 17,614
2019	17,834
2020	18,174
2021	18,635
2022	19,235
Next 5 years	83,071
Total	174,563
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
2018	12,381
2019	12,549
2020	12,990
2021	13,239
2022	13,423
Next 5 years	62,854
Total	$ 127,436