Employee Benefit Plans - Summary of funded status of Retirement Income Plan and amounts recognized in consolidated balance sheets (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
CHANGE IN PLAN ASSETS:
Funded status at end of year	$ (13,180)	$ (14,098)
AMOUNTS (PRE-TAX) RECOGNIZED IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) CONSIST OF:
Pension adjustment, Before-tax amount	(2,688)	(4,800)
Retirement Income Plan
Defined Benefit Plan Disclosure [Line Items]
Accumulated Benefit Obligation at end of year	42,699	38,278
CHANGE IN PROJECTED BENEFIT OBLIGATION:
Benefit obligation at beginning of year	38,278	35,873
Service cost
Interest cost	1,869	1,916	1,893
Amendments
Actuarial loss	4,221	2,123
Benefits paid	(1,669)	(1,634)
Projected benefit obligation at end of year	42,699	38,278	35,873
CHANGE IN PLAN ASSETS:
Fair value of plan assets at beginning of year	24,180	26,523
Actual return on plan assets	2,712	(1,309)
Employer contribution	4,296	600
Benefits paid	(1,669)	(1,634)
Fair value of plan assets at end of year	29,519	24,180	26,523
Funded status at end of year	(13,180)	(14,098)
AMOUNTS RECOGNIZED IN THE CONSOLIDATED BALANCE SHEETS CONSIST OF:
Noncurrent assets
Current liabilities
Noncurrent liabilities	(13,180)	(14,098)
Amounts recognized in balance sheet	(13,180)	(14,098)
AMOUNTS (PRE-TAX) RECOGNIZED IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) CONSIST OF:
Net loss (gain)	23,129	20,441
Prior service cost (credit)
Net transition obligation (asset)
Pension adjustment, Before-tax amount	$ 23,129	$ 20,441