Benefit Plans - Fair Value of Plan Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	$ 401,142	$ 360,900
Pension Plan | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	24,736	24,260
Pension Plan | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	376,406	336,640
Pension Plan | Domestic Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	1,006	11,805
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	1,006	11,805
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 Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	202,840	178,412
Pension Plan | Corporate Bonds | 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 Bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	202,840	178,412
Pension Plan | U.S. Government Securities
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	19,396	7,115
Pension Plan | U.S. Government Securities | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	17,996	6,761
Pension Plan | U.S. Government Securities | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	1,400	354
Pension Plan | Municipal Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	62,293	63,492
Pension Plan | Municipal Bonds | 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 | Municipal Bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	62,293	63,492
Pension Plan | Foreign Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	32,949	27,917
Pension Plan | Foreign Bonds | 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 Bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	32,949	27,917
Pension Plan | Exchange traded fund
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	5,734	5,694
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	5,734	5,694
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	76,924	66,465
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	76,924	66,465
Other Postretirement Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	85,303	70,408	$ 65,568
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	85,303	70,408
Other Postretirement Benefit Plan | Domestic Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	64,489	54,426
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	64,489	54,426
Other Postretirement Benefit Plan | International Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	20,814	15,982
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	$ 20,814	$ 15,982