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Pension and Postretirement Plans (Narrative) (Details) - Defined Benefit Pension Plan [Member] $ in Thousands | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jun. 30, 2019 USD ($) Investment companies | Jun. 30, 2018 USD ($) | Jun. 30, 2019 USD ($) Investment companies | Jun. 30, 2018 USD ($) | Dec. 31, 2018 USD ($) Investment
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Special separation benefit expense | $ 6,607 | $ 0 | $ 6,607 | $ 0 |
Percent of plan assets managed internally by the company |  |  | 44.00% |  |
Percent of plan assets managed by investment companies |  |  | 56.00% |  |
Number of investment companies actively managing plan assets | companies | 2 |  | 2 |  |
Percentage of total plan assets | 100.00% |  | 100.00% |  | 100.00%
Berkshire Hathaway Common Stock [Member] | Investment Manager 2 [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Defined benefit plan, target allocation maximum percentage of assets, singular equity security, without prior approval by plan administrator |  |  | 20.00% |  |
Alphabet and Berkshire Hathaway Common Stock [Member] | Investment Manager 1 [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Defined benefit plan, target allocation maximum percentage of assets, singular equity security, without prior approval by plan administrator |  |  | 15.00% |  |
Foreign Investments [Member] | Maximum [Member] | Investment Manager 1 [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Defined benefit plan, target allocation percentage of assets | 25.00% |  | 25.00% |  |
Foreign Investments [Member] | Maximum [Member] | Investment Manager 2 [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Defined benefit plan, target allocation percentage of assets | 15.00% |  | 15.00% |  |
Fixed income securities [Member] | Minimum [Member] | Investment Manager 1 [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Defined benefit plan, target allocation percentage of assets | 5.00% |  | 5.00% |  |
Fixed income securities [Member] | Minimum [Member] | Investment Manager 2 [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Defined benefit plan, target allocation percentage of assets | 10.00% |  | 10.00% |  |
Single Equity Concentration [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Defined benefit plan, target allocation maximum percentage of assets, singular equity security, without prior approval by plan administrator |  |  | 10.00% |  |
Value of investments | $ 1,000,100 |  | $ 1,000,100 |  | $ 945,600
Percentage of total plan assets | 41.00% |  | 41.00% |  | 45.00%
Single Equity Concentration [Member] | Equity Securities [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Number of investments the company's pension plan held which individually exceed 10% of total plan assets | Investment | 1 |  | 1 |  | 1
Single Equity Concentration [Member] | Equity Funds [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Number of investments the company's pension plan held which individually exceed 10% of total plan assets | Investment | 1 |  | 1 |  | 1
Concentration In Single Entity, Type Of Industry, Foreign Country Or Individual Fund [Member] | Defined Benefit Plan Assets Total [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Minimum percentage of plan assets considered as significant concentrations in pension plans |  |  | 10.00% |  | 10.00%
Education [Member] | Separation Incentive Program [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Special separation benefit expense | $ 6,600 |  |  |  |

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