Benefit Plans - Fair Value of Plan Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	$ 350,107	$ 378,701
Pension Plan | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	10,251	20,876
Pension Plan | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	339,856	357,825
Pension Plan | Domestic Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	1,442	9,913
Pension Plan | Domestic Mutual Funds | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	1,442	9,913
Pension Plan | Domestic Mutual Funds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0	0
Pension Plan | Corporate Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	188,332	200,732
Pension Plan | Corporate Debt Securities | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0	0
Pension Plan | Corporate Debt Securities | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	188,332	200,732
Pension Plan | U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	3,630	6,561
Pension Plan | U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	3,133	5,327
Pension Plan | U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	497	1,234
Pension Plan | Obligations of U.S. States and Political Subdivisions
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	61,206	65,214
Pension Plan | Obligations of U.S. States and Political Subdivisions | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0	0
Pension Plan | Obligations of U.S. States and Political Subdivisions | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	61,206	65,214
Pension Plan | Foreign Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	25,251	23,028
Pension Plan | Foreign Debt Securities | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0	0
Pension Plan | Foreign Debt Securities | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	25,251	23,028
Pension Plan | Exchange traded fund
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	5,676	5,636
Pension Plan | Exchange traded fund | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	5,676	5,636
Pension Plan | Exchange traded fund | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0	0
Pension Plan | Pooled Equity Accounts
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	64,570	67,617
Pension Plan | Pooled Equity Accounts | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0	0
Pension Plan | Pooled Equity Accounts | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	64,570	67,617
Other Postretirement Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	65,568	66,940	$ 62,298
Other Postretirement Benefit Plan | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	65,568	66,940
Other Postretirement Benefit Plan | Domestic Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	49,887	50,710
Other Postretirement Benefit Plan | Domestic Mutual Funds | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	49,887	50,710
Other Postretirement Benefit Plan | International Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	15,681	16,230
Other Postretirement Benefit Plan | International Mutual Funds | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	$ 15,681	$ 16,230