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, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation at end of year	$ 46,397	$ 44,315
CHANGE IN PROJECTED BENEFIT OBLIGATION:
Benefit obligation at beginning of year	44,315	42,894
Service cost	0	0	$ 0
Interest cost	1,932	2,006	1,898
Amendments	0	0
Actuarial loss	2,206	1,371
Benefits paid	(2,056)	(1,956)
Projected benefit obligation at end of year	46,397	44,315	42,894
CHANGE IN PLAN ASSETS:
Fair value of plan assets at beginning of year	34,745	30,937
Actual return on plan assets	5,361	1,464
Employer contribution	0	4,300
Benefits paid	(2,056)	(1,956)
Fair value of plan assets at end of year	38,050	34,745	30,937
Funded status at end of year	(8,347)	(9,570)
AMOUNTS (PRE-TAX) RECOGNIZED IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) CONSIST OF:
Net loss	22,762	24,412
Prior service cost (credit)	0	0
Net transition obligation (asset)	0	0	$ 0
Before-tax amount	$ 22,762	$ 24,412