Employee Benefit Plans - Summary of plan assets using fair value hierarchy (Details) (Retirement Income Plan, USD $) In Thousands, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 29,519		$ 24,180		$ 26,523
Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	29,519		24,180
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	11,187		9,094
Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	15,602		13,736
Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,730		1,350
Cash and Cash Equivalents | Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	61	[1]	175	[1]
Cash and Cash Equivalents | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	61	[1]	175	[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	5,959	[2]	5,608	[2]
Fixed Income Securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		[2]		[2]
Fixed Income Securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,959	[2]	5,608	[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	4,475		3,680
Domestic Equity Securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,475		3,680
Domestic Equity Securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Domestic Equity Securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Global Equity Securities | Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,446		3,568
Global Equity Securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,446		3,568
Global Equity Securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Global Equity Securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
International Equity Securities | Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,737	[3]	3,537	[3]
International Equity Securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,205	[3]	1,671	[3]
International Equity Securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,532	[3]	1,866	[3]
International Equity Securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		[3]		[3]
Tactical Composite | Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,454	[4]	3,935	[4]
Tactical Composite | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		[4]		[4]
Tactical Composite | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,454	[4]	3,935	[4]
Tactical Composite | 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	2,730	[5]	1,350	[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	2,730	[5]	1,350	[5]
Real Return | Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,657	[6]	2,327	[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	2,657	[6]	2,327	[6]
Real Return | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		[6]		[6]

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

[4] Tactical composite funds invest in stocks, bonds and cash, both domestic and international. These assets are valued primarily 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.