Employee Benefit Plans - Summary of plan assets using fair value hierarchy (Details 7) - Retirement Income Plan - USD ($) $ in Thousands		Dec. 31, 2018	Dec. 31, 2017
Total
Defined Benefit Plan Disclosure [Line Items]
Investments measured at Net Asset Value		$ 3,600	$ 3,910
Investments at Fair Value		38,364	38,050
Cash and Cash Equivalents | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Assets in the Fair Value Hierarchy	[1]	1,140	1,084
Cash and Cash Equivalents | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Assets in the Fair Value Hierarchy	[1]	0	0
Cash and Cash Equivalents | Total
Defined Benefit Plan Disclosure [Line Items]
Total Assets in the Fair Value Hierarchy	[1]	1,140	1,084
Fixed Income Securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Assets in the Fair Value Hierarchy	[2]	0	0
Fixed Income Securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Assets in the Fair Value Hierarchy	[2]	11,163	9,064
Fixed Income Securities | Total
Defined Benefit Plan Disclosure [Line Items]
Total Assets in the Fair Value Hierarchy	[2]	11,163	9,064
Domestic Equity Securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Assets in the Fair Value Hierarchy	[3]	5,602	5,930
Domestic Equity Securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Assets in the Fair Value Hierarchy	[3]	9,581	10,173
Domestic Equity Securities | Total
Defined Benefit Plan Disclosure [Line Items]
Total Assets in the Fair Value Hierarchy	[3]	15,182	16,103
International Equity Securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Assets in the Fair Value Hierarchy	[4]	0	0
International Equity Securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Assets in the Fair Value Hierarchy	[4]	7,279	7,889
International Equity Securities | Total
Defined Benefit Plan Disclosure [Line Items]
Total Assets in the Fair Value Hierarchy	[4]	7,279	7,889
Investment
Defined Benefit Plan Disclosure [Line Items]
Total Assets in the Fair Value Hierarchy		34,764	34,140
Investment | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Assets in the Fair Value Hierarchy		6,742	7,014
Investment | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Assets in the Fair Value Hierarchy		$ 28,023	$ 27,126

[1] Cash and cash equivalents, which are used to pay benefits and plan administrative expenses, are held in Rule 2a-7 money market funds.

[2] Fixed income securities are primarily valued using a market approach with inputs that include broker quotes, benchmark yields, base spreads and reported trades.

[3] Domestic equity securities are valued using a market approach based on the quoted market prices of identical instruments in their respective markets.

[4] International equity securities are valued using a market approach based on the quoted market prices of identical instruments in their respective markets.