Employee Benefit Plans - Summary of plan assets using fair value hierarchy (Details 7) - Retirement Income Plan - USD ($) $ in Thousands		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		$ 34,745	$ 30,937	$ 32,622
Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		26,038	22,635
Investments measured at Net Asset Value		8,707	8,302
Investments at Fair Value		34,745	30,937
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		5,623	4,495
Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		20,415	18,140
Cash and Cash Equivalents | Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1]	1,154	210
Cash and Cash Equivalents | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1]	1,154	210
Cash and Cash Equivalents | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1]
Fixed Income Securities | Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[2]	8,804	7,987
Fixed Income Securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[2]
Fixed Income Securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[2]	8,804	7,987
Domestic Equity Securities | Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[3]	8,865	8,527
Domestic Equity Securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[3]	4,469	4,285
Domestic Equity Securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[3]	4,396	4,242
International Equity Securities | Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[4]	7,215	5,911
International Equity Securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[4]
International Equity Securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[4]	$ 7,215	$ 5,911

[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.