Employee Benefits (Fair values of retirement plan assets) (Details 5) - Pension Benefits - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined benefit pension plan disclosure
Fair value of plan assets	$ 96,908	$ 83,383	$ 83,858
Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined benefit pension plan disclosure
Fair value of plan assets	96,908
Cash
Defined benefit pension plan disclosure
Fair value of plan assets	2,156	1,898
Cash | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined benefit pension plan disclosure
Fair value of plan assets	2,156
Equity Security - Mutual Funds: Large-Cap
Defined benefit pension plan disclosure
Fair value of plan assets	1,748	0
Equity Security - Mutual Funds: Large-Cap | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined benefit pension plan disclosure
Fair value of plan assets	1,748
Equity Security - Mutual Funds: Mixed-Cap
Defined benefit pension plan disclosure
Fair value of plan assets	29,459	29,593
Equity Security - Mutual Funds: Mixed-Cap | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined benefit pension plan disclosure
Fair value of plan assets	29,459
Equity Security - Mutual Funds: International
Defined benefit pension plan disclosure
Fair value of plan assets	24,539	18,040
Equity Security - Mutual Funds: International | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined benefit pension plan disclosure
Fair value of plan assets	24,539
Equity Security - Mutual Funds: Emerging Market
Defined benefit pension plan disclosure
Fair value of plan assets	2,305	2,043
Equity Security - Mutual Funds: Emerging Market | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined benefit pension plan disclosure
Fair value of plan assets	2,305
Fixed Income - Mutual Funds
Defined benefit pension plan disclosure
Fair value of plan assets	36,701	$ 31,809
Fixed Income - Mutual Funds | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined benefit pension plan disclosure
Fair value of plan assets	$ 36,701