Employee Benefit Plans - Summary of funded status of Retirement Income Plan and amounts recognized in consolidated balance sheets (Details) (Retirement Income Plan, USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Retirement Income Plan
Defined Benefit Plan Disclosure [Line Items]
Accumulated Benefit Obligation at end of year	$ 47,410	$ 37,528
CHANGE IN PROJECTED BENEFIT OBLIGATION:
Benefit obligation at beginning of year	37,528	42,699
Service cost
Interest cost	1,946	1,741	1,869
Amendments
Actuarial loss (gain)	9,725	(5,199)
Benefits paid	(1,789)	(1,713)
Projected benefit obligation at end of year	47,410	37,528	42,699
CHANGE IN PLAN ASSETS:
Fair value of plan assets at beginning of year	32,426	29,519
Actual return on plan assets	1,220	3,820
Employer contribution	765	800
Benefits paid	(1,789)	(1,713)
Fair value of plan assets at end of year	32,622	32,426	29,519
Funded status at end of year	(14,788)	(5,102)
AMOUNTS RECOGNIZED IN THE CONSOLIDATED BALANCE SHEETS CONSIST OF:
Noncurrent assets
Current liabilities
Noncurrent liabilities	(14,788)	(5,102)
Amounts recognized in balance sheet	(14,788)	(5,102)
AMOUNTS (PRE-TAX) RECOGNIZED IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) CONSIST OF:
Net loss (gain)	25,583	15,369
Prior service cost (credit)
Net transition obligation (asset)
Before-tax amount	$ 25,583	$ 15,369