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, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation at end of year	$ 44,315	$ 42,894
CHANGE IN PROJECTED BENEFIT OBLIGATION:
Benefit obligation at beginning of year	42,894	47,410
Service cost
Interest cost	2,006	1,898	1,946
Amendments
Actuarial loss (gain)	1,371	(4,593)
Benefits paid	(1,956)	(1,821)
Projected benefit obligation at end of year	44,315	42,894	47,410
CHANGE IN PLAN ASSETS:
Fair value of plan assets at beginning of year	30,937	32,622
Actual return on plan assets	1,464	(714)
Employer contribution	4,300	850
Benefits paid	(1,956)	(1,821)
Fair value of plan assets at end of year	34,745	30,937	32,622
Funded status at end of year	(9,570)	(11,957)
AMOUNTS (PRE-TAX) RECOGNIZED IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) CONSIST OF:
Net loss (gain)	24,412	23,172
Prior service cost (credit)
Net transition obligation (asset)
Before-tax amount	$ 24,412	$ 23,172