Employee Benefit Plans - Summary of plan assets using fair value hierarchy (Details 7) - Retirement Income Plan - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Total Assets in the Fair Value Hierarchy		$ 38,050	$ 34,745	$ 30,937
Total
Defined Benefit Plan Disclosure [Line Items]
Total Assets in the Fair Value Hierarchy		34,140	26,038
Investments measured at Net Asset Value		3,910	8,707
Investments at Fair Value		38,050	34,745
Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Assets in the Fair Value Hierarchy		7,014	5,623
Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Assets in the Fair Value Hierarchy		27,126	20,415
Cash and Cash Equivalents | Total
Defined Benefit Plan Disclosure [Line Items]
Total Assets in the Fair Value Hierarchy	[1]	1,084	1,154
Cash and Cash Equivalents | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Assets in the Fair Value Hierarchy	[1]	1,084	1,154
Cash and Cash Equivalents | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Assets in the Fair Value Hierarchy	[1]	0	0
Fixed Income Securities | Total
Defined Benefit Plan Disclosure [Line Items]
Total Assets in the Fair Value Hierarchy	[2]	9,064	8,804
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]	9,064	8,804
Domestic Equity Securities | Total
Defined Benefit Plan Disclosure [Line Items]
Total Assets in the Fair Value Hierarchy	[3]	16,103	8,865
Domestic Equity Securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Assets in the Fair Value Hierarchy	[3]	5,930	4,469
Domestic Equity Securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Assets in the Fair Value Hierarchy	[3]	10,173	4,396
International Equity Securities | Total
Defined Benefit Plan Disclosure [Line Items]
Total Assets in the Fair Value Hierarchy	[4]	7,889	7,215
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]	$ 7,889	$ 7,215

[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.