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	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation at end of year	$ 42,894	$ 47,410
CHANGE IN PROJECTED BENEFIT OBLIGATION:
Benefit obligation at beginning of year	$ 47,410	$ 37,528
Service cost
Interest cost	$ 1,898	$ 1,946	$ 1,741
Amendments
Actuarial loss (gain)	$ (4,593)	$ 9,725
Benefits paid	(1,821)	(1,789)
Projected benefit obligation at end of year	42,894	47,410	37,528
CHANGE IN PLAN ASSETS:
Fair value of plan assets at beginning of year	32,622	32,426
Actual return on plan assets	(714)	1,220
Employer contribution	850	765
Benefits paid	(1,821)	(1,789)
Fair value of plan assets at end of year	30,937	32,622	$ 32,426
Funded status at end of year	$ (11,957)	$ (14,788)
AMOUNTS RECOGNIZED IN THE CONSOLIDATED BALANCE SHEETS CONSIST OF:
Noncurrent assets
Current liabilities
Noncurrent liabilities	$ (11,957)	$ (14,788)
Amounts recognized in balance sheet	(11,957)	(14,788)
AMOUNTS (PRE-TAX) RECOGNIZED IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) CONSIST OF:
Net loss (gain)	$ 23,172	$ 25,583
Prior service cost (credit)
Net transition obligation (asset)
Before-tax amount	$ 23,172	$ 25,583