Employee Benefit Plans - Defined Benefit Plans - Components of Periodic Pension Costs (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other changes in plan assets and benefit obligations recognized in other comprehensive (income) loss
Amortization of prior service cost arising during the period	[1]	$ 61	$ 60	$ 61
Total recognized in other comprehensive (income) loss		157	(123)	(513)
The Bank | Cash Balance Plan
Components of net periodic benefit cost
Service cost		1,120	1,076	1,040
Interest cost		552	528	468
Expected return on plan assets		(1,138)	(1,045)	(1,043)
Amortization of prior service cost		(61)	(61)	(61)
Recognized net actuarial loss		154	158	130
Net periodic benefit cost		627	656	534
Other changes in plan assets and benefit obligations recognized in other comprehensive (income) loss
Net (gain) loss		(303)	129	602
Prior service cost				126
Amortization of prior service cost arising during the period		61	60	61
Deferred taxes		85	(66)	(276)
Total recognized in other comprehensive (income) loss		(157)	123	513
Total recognized in net periodic benefit cost and other comprehensive loss (income )		$ 470	$ 779	$ 1,047

[1] These items are included in the computation of net periodic benefit cost, which is a component of “Salaries and employee benefits” on the consolidated statements of income. See “Note 12: Employee Benefit Plans,” for additional information.