Benefit Plans Fair Value of Plan Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 140,164	$ 119,261
Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	88,990	73,060
Significant Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	51,174	46,201
Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8,207	10,050
Cash and Cash Equivalents [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8,207	10,050
Cash and Cash Equivalents [Member] | Significant Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Cash and Cash Equivalents [Member] | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 0	0
Fixed Income Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations	25.00%
Fair value of plan assets	$ 44,994	40,514
Fixed Income Funds [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fixed Income Funds [Member] | Significant Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	44,994	40,514
Fixed Income Funds [Member] | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 0	0
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations	55.00%
Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations	45.00%
All Cap Equity Fund [Member] | Actively Managed Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 6,413	5,290
All Cap Equity Fund [Member] | Actively Managed Investments [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,376	5,266
All Cap Equity Fund [Member] | Actively Managed Investments [Member] | Significant Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	37	24
All Cap Equity Fund [Member] | Actively Managed Investments [Member] | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.S. Large Cap Equity [Member] | Actively Managed Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	50,351	39,534
U.S. Large Cap Equity [Member] | Actively Managed Investments [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	50,351	39,534
U.S. Large Cap Equity [Member] | Actively Managed Investments [Member] | Significant Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.S. Large Cap Equity [Member] | Actively Managed Investments [Member] | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.S. Mid Cap Equity [Member] | Actively Managed Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	9,358	7,021
U.S. Mid Cap Equity [Member] | Actively Managed Investments [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	9,358	7,021
U.S. Mid Cap Equity [Member] | Actively Managed Investments [Member] | Significant Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.S. Mid Cap Equity [Member] | Actively Managed Investments [Member] | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.S. Small Cap Equity [Member] | Actively Managed Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8,725	6,357
U.S. Small Cap Equity [Member] | Actively Managed Investments [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8,725	6,357
U.S. Small Cap Equity [Member] | Actively Managed Investments [Member] | Significant Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.S. Small Cap Equity [Member] | Actively Managed Investments [Member] | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Non-U.S. Large Cap Equity [Member] | Actively Managed Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,973	4,832
Non-U.S. Large Cap Equity [Member] | Actively Managed Investments [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,973	4,832
Non-U.S. Large Cap Equity [Member] | Actively Managed Investments [Member] | Significant Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Non-U.S. Large Cap Equity [Member] | Actively Managed Investments [Member] | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
REIT [Member] | Actively Managed Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,143	5,663
REIT [Member] | Actively Managed Investments [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
REIT [Member] | Actively Managed Investments [Member] | Significant Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,143	5,663
REIT [Member] | Actively Managed Investments [Member] | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 0	$ 0