Benefit Plans - Fair Value of Plan Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	$ 402,691	$ 359,719
Pension Plan | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	39,491	38,889
Pension Plan | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	363,200	320,830
Pension Plan | Domestic Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	7,325	13,744
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	7,325	13,744
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	203,684	181,363
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	203,684	181,363
Pension Plan | U.S. Government Securities
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	34,677	21,228
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	32,166	19,904
Pension Plan | U.S. Government Securities | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	2,511	1,324
Pension Plan | Municipal Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	38,998	43,424
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	38,998	43,424
Pension Plan | Foreign Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	34,024	30,113
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	34,024	30,113
Pension Plan | Exchange traded fund
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0	5,241
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	0	5,241
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	83,983	64,606
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	83,983	64,606
Other Postretirement Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	99,590	77,762	$ 85,303
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	99,590	77,762
Other Postretirement Benefit Plan | Domestic Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	77,640	60,405
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	77,640	60,405
Other Postretirement Benefit Plan | International Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	21,950	17,357
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	$ 21,950	$ 17,357