Employee Benefit Plans (Fair Value of Plan Assets) (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 |
Pension Plans, Defined Benefit [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | $ 719,332 | $ 658,474 | $ 653,945 |
Pension Plans, Defined Benefit [Member] | Level 1 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Pension Plans, Defined Benefit [Member] | Level 2 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 612,109 | 558,818 | |
Pension Plans, Defined Benefit [Member] | Level 3 | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Pension Plans, Defined Benefit [Member] | Domestic Equity Securities [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 212,746 | 191,987 | |
Pension Plans, Defined Benefit [Member] | Domestic Equity Securities [Member] | Level 1 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Pension Plans, Defined Benefit [Member] | Domestic Equity Securities [Member] | Level 2 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 188,850 | 168,407 | |
Pension Plans, Defined Benefit [Member] | Domestic Equity Securities [Member] | Level 3 | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Pension Plans, Defined Benefit [Member] | International Equity Securities [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 98,646 | 83,738 | |
Pension Plans, Defined Benefit [Member] | International Equity Securities [Member] | Level 1 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Pension Plans, Defined Benefit [Member] | International Equity Securities [Member] | Level 2 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 98,646 | 83,738 | |
Pension Plans, Defined Benefit [Member] | International Equity Securities [Member] | Level 3 | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Pension Plans, Defined Benefit [Member] | Emerging market equity fund [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 26,804 | 21,055 | |
Pension Plans, Defined Benefit [Member] | Emerging market equity fund [Member] | Level 1 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Pension Plans, Defined Benefit [Member] | Emerging market equity fund [Member] | Level 2 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 26,804 | 21,055 | |
Pension Plans, Defined Benefit [Member] | Emerging market equity fund [Member] | Level 3 | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Pension Plans, Defined Benefit [Member] | Domestic Bond Fund [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 100,687 | 101,200 | |
Pension Plans, Defined Benefit [Member] | Domestic Bond Fund [Member] | Level 1 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Pension Plans, Defined Benefit [Member] | Domestic Bond Fund [Member] | Level 2 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 100,687 | 101,200 | |
Pension Plans, Defined Benefit [Member] | Domestic Bond Fund [Member] | Level 3 | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Pension Plans, Defined Benefit [Member] | Core Bonds [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 98,874 | 86,109 | |
Pension Plans, Defined Benefit [Member] | Core Bonds [Member] | Level 1 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Pension Plans, Defined Benefit [Member] | Core Bonds [Member] | Level 2 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 98,874 | 86,109 | |
Pension Plans, Defined Benefit [Member] | Core Bonds [Member] | Level 3 | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Pension Plans, Defined Benefit [Member] | High-Yield Bonds [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 31,692 | 30,729 | |
Pension Plans, Defined Benefit [Member] | High-Yield Bonds [Member] | Level 1 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Pension Plans, Defined Benefit [Member] | High-Yield Bonds [Member] | Level 2 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 31,692 | 30,729 | |
Pension Plans, Defined Benefit [Member] | High-Yield Bonds [Member] | Level 3 | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Pension Plans, Defined Benefit [Member] | Emerging Market Bonds [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 25,959 | 23,584 | |
Pension Plans, Defined Benefit [Member] | Emerging Market Bonds [Member] | Level 1 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Pension Plans, Defined Benefit [Member] | Emerging Market Bonds [Member] | Level 2 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 25,959 | 23,584 | |
Pension Plans, Defined Benefit [Member] | Emerging Market Bonds [Member] | Level 3 | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Pension Plans, Defined Benefit [Member] | Combination Debt Equity Fund Member | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 36,167 | 37,851 | |
Pension Plans, Defined Benefit [Member] | Combination Debt Equity Fund Member | Level 1 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Pension Plans, Defined Benefit [Member] | Combination Debt Equity Fund Member | Level 2 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 36,167 | 37,851 | |
Pension Plans, Defined Benefit [Member] | Combination Debt Equity Fund Member | Level 3 | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Pension Plans, Defined Benefit [Member] | Real Estate [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 34,421 | 32,390 | |
Pension Plans, Defined Benefit [Member] | Real Estate [Member] | Level 1 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Pension Plans, Defined Benefit [Member] | Real Estate [Member] | Level 2 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Pension Plans, Defined Benefit [Member] | Real Estate [Member] | Level 3 | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Pension Plans, Defined Benefit [Member] | Alternative Investments [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 48,906 | 43,686 | |
Pension Plans, Defined Benefit [Member] | Alternative Investments [Member] | Level 1 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Pension Plans, Defined Benefit [Member] | Alternative Investments [Member] | Level 2 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Pension Plans, Defined Benefit [Member] | Alternative Investments [Member] | Level 3 | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Pension Plans, Defined Benefit [Member] | Cash Equivalents [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 4,430 | 6,145 | |
Pension Plans, Defined Benefit [Member] | Cash Equivalents [Member] | Level 1 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Pension Plans, Defined Benefit [Member] | Cash Equivalents [Member] | Level 2 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 4,430 | 6,145 | |
Pension Plans, Defined Benefit [Member] | Cash Equivalents [Member] | Level 3 | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Other Postretirement Benefit Plans, Defined Benefit [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 124,070 | 115,619 | $ 115,416 |
Other Postretirement Benefit Plans, Defined Benefit [Member] | Level 1 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Other Postretirement Benefit Plans, Defined Benefit [Member] | Level 2 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 124,070 | 115,619 | |
Other Postretirement Benefit Plans, Defined Benefit [Member] | Level 3 | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Other Postretirement Benefit Plans, Defined Benefit [Member] | Domestic Equity Securities [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 65,187 | 61,055 | |
Other Postretirement Benefit Plans, Defined Benefit [Member] | Domestic Equity Securities [Member] | Level 1 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Other Postretirement Benefit Plans, Defined Benefit [Member] | Domestic Equity Securities [Member] | Level 2 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 65,187 | 61,055 | |
Other Postretirement Benefit Plans, Defined Benefit [Member] | Domestic Equity Securities [Member] | Level 3 | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Other Postretirement Benefit Plans, Defined Benefit [Member] | International Equity Securities [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 16,217 | 15,034 | |
Other Postretirement Benefit Plans, Defined Benefit [Member] | International Equity Securities [Member] | Level 1 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Other Postretirement Benefit Plans, Defined Benefit [Member] | International Equity Securities [Member] | Level 2 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 16,217 | 15,034 | |
Other Postretirement Benefit Plans, Defined Benefit [Member] | International Equity Securities [Member] | Level 3 | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Other Postretirement Benefit Plans, Defined Benefit [Member] | Core Bonds [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 42,083 | 38,952 | |
Other Postretirement Benefit Plans, Defined Benefit [Member] | Core Bonds [Member] | Level 1 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Other Postretirement Benefit Plans, Defined Benefit [Member] | Core Bonds [Member] | Level 2 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 42,083 | 38,952 | |
Other Postretirement Benefit Plans, Defined Benefit [Member] | Core Bonds [Member] | Level 3 | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Other Postretirement Benefit Plans, Defined Benefit [Member] | Cash Equivalents [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 583 | 578 | |
Other Postretirement Benefit Plans, Defined Benefit [Member] | Cash Equivalents [Member] | Level 1 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Other Postretirement Benefit Plans, Defined Benefit [Member] | Cash Equivalents [Member] | Level 2 [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 583 | 578 | |
Other Postretirement Benefit Plans, Defined Benefit [Member] | Cash Equivalents [Member] | Level 3 | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Portion at Other than Fair Value Measurement [Member] | Pension Plans, Defined Benefit [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 107,223 | 99,656 | |
Portion at Other than Fair Value Measurement [Member] | Pension Plans, Defined Benefit [Member] | Domestic Equity Securities [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 23,896 | 23,580 | |
Portion at Other than Fair Value Measurement [Member] | Pension Plans, Defined Benefit [Member] | International Equity Securities [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Portion at Other than Fair Value Measurement [Member] | Pension Plans, Defined Benefit [Member] | Emerging market equity fund [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Portion at Other than Fair Value Measurement [Member] | Pension Plans, Defined Benefit [Member] | Domestic Bond Fund [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Portion at Other than Fair Value Measurement [Member] | Pension Plans, Defined Benefit [Member] | Core Bonds [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Portion at Other than Fair Value Measurement [Member] | Pension Plans, Defined Benefit [Member] | High-Yield Bonds [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Portion at Other than Fair Value Measurement [Member] | Pension Plans, Defined Benefit [Member] | Emerging Market Bonds [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Portion at Other than Fair Value Measurement [Member] | Pension Plans, Defined Benefit [Member] | Combination Debt Equity Fund Member | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Portion at Other than Fair Value Measurement [Member] | Pension Plans, Defined Benefit [Member] | Real Estate [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 48,906 | 43,686 | |
Portion at Other than Fair Value Measurement [Member] | Pension Plans, Defined Benefit [Member] | Alternative Investments [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 34,421 | 32,390 | |
Portion at Other than Fair Value Measurement [Member] | Pension Plans, Defined Benefit [Member] | Cash Equivalents [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Portion at Other than Fair Value Measurement [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Portion at Other than Fair Value Measurement [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Domestic Equity Securities [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Portion at Other than Fair Value Measurement [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | International Equity Securities [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Portion at Other than Fair Value Measurement [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Core Bonds [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | 0 | 0 | |
Portion at Other than Fair Value Measurement [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Cash Equivalents [Member] | | | |
Defined Benefit Plan Disclosure [Line Items] | | | |
Defined Benefit Plan, Fair Value of Plan Assets | $ 0 | $ 0 | |