Employee Benefit Plans - Summary of plan assets using fair value hierarchy (Details 7) (Retirement Income Plan, USD $) In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 32,622		$ 32,426		$ 29,519
Cash and Cash Equivalents | Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	329	[1]	210	[1]
Cash and Cash Equivalents | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	329	[1]	210	[1]
Cash and Cash Equivalents | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		[1]		[1]
Cash and Cash Equivalents | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		[1]		[1]
Fixed Income Securities | Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7,915	[2]	8,201	[2]
Fixed Income Securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,194	[2]		[2]
Fixed Income Securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,721	[2]	8,201	[2]
Fixed Income Securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		[2]		[2]
Domestic Equity Securities | Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	12,076	[3]	8,590	[3]
Domestic Equity Securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,324	[3]	8,590	[3]
Domestic Equity Securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7,752	[3]		[3]
Domestic Equity Securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		[3]		[3]
International Equity Securities | Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7,442	[4]	10,192	[4]
International Equity Securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		[4]		[4]
International Equity Securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7,442	[4]	10,192	[4]
International Equity Securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		[4]		[4]
Real Estate | Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,420	[5]	2,705	[5]
Real Estate | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		[5]		[5]
Real Estate | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		[5]		[5]
Real Estate | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,420	[5]	2,705	[5]
Real Return | Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	515	[6]	2,528	[6]
Real Return | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		[6]		[6]
Real Return | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	515	[6]	2,528	[6]
Real Return | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		[6]		[6]
Alternative/Opportunistic/Special funds | Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	925	[7]
Alternative/Opportunistic/Special funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		[7]
Alternative/Opportunistic/Special funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		[7]
Alternative/Opportunistic/Special funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 925	[7]

[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.

[5] Real estate fund values are primarily reported by the fund manager and are based on valuation of the underlying investments, which include inputs such as cost, discounted future cash flows, independent appraisals and market based comparable data.

[6] Real return funds invest in global equities, commodities and inflation protected core bonds that are valued primarily using a market approach based on the quoted market prices of identical instruments in their respective markets.

[7] Alternative/Opportunistic/Special funds can invest across the capital structure in both liquid and illiquid securities that are valued using a market approach based on the quoted market prices of identical instruments, or if no market price is available, instruments will be held at their fair market value (which may be cost) as reasonably determined by the investment manager, independent dealers, or pricing services.