Pension Plans and Other Postretirement Benefits - Defined Benefit Plan and Supplemental Retirement Plans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan
Change in benefit obligation:
Benefit obligations at beginning of year	$ 29,407	$ 31,233	$ 34,279
Service cost	650	650	450
Interest cost	1,091	1,244	1,189
Plan amendments	0	0	0
Actuarial loss (gain)	721	220	1,371
Benefits paid	(1,995)	(3,500)	(3,314)
Settlements	0	0	0
Benefit obligations at beginning of year	28,432	29,407	31,233
Change in plan assets:
Fair value of plan assets at beginning of year	25,446	27,667	31,408
Actual return on plan assets	2,909	1,766	200
Employer contributions	0	0	0
Benefits paid	(1,995)	(3,500)	(3,314)
Expenses paid	(517)	(487)	(627)
Settlements	0	0	0
Fair value of plan assets at end of year	25,843	25,446	27,667
Funded status	2,589	3,961	3,566
Prepaid (accrued) benefit expense	8,016	8,653	9,265
Total amount recognized in Consolidated Balance Sheets, all in Other liabilities	(2,589)	(3,961)	(3,566)
Amounts recognized in accumulated other comprehensive income (loss) ("AOCI"):
Prior service cost	0	0	0
Net actuarial loss	10,605	12,613	12,831
Total amount recognized in AOCI	10,605	12,613	12,831
Information for pension plans with an accumulated benefit obligation greater than plan assets:
Projected benefit obligation	28,432	29,407	31,233
Accumulated benefit obligation	28,432	29,407	31,233
Fair value of plan assets	25,843	25,446	27,667
Supplemental Defined Benefit Plans
Change in benefit obligation:
Benefit obligations at beginning of year	16,847	17,004	18,524
Service cost	0	0	0
Interest cost	631	687	654
Plan amendments	0	0	0
Actuarial loss (gain)	(805)	(488)	845
Benefits paid	(1,451)	(1,332)	(1,329)
Settlements	0	0	0
Benefit obligations at beginning of year	16,832	16,847	17,004
Change in plan assets:
Fair value of plan assets at beginning of year	0	0	0
Actual return on plan assets	0	0	0
Employer contributions	1,451	1,332	1,329
Benefits paid	(1,451)	(1,332)	(1,329)
Expenses paid	0	0	0
Settlements	0	0	0
Fair value of plan assets at end of year	0	0	0
Funded status	16,832	16,847	17,004
Prepaid (accrued) benefit expense	(10,648)	(11,210)	(11,622)
Total amount recognized in Consolidated Balance Sheets, all in Other liabilities	(16,832)	(16,847)	(17,004)
Amounts recognized in accumulated other comprehensive income (loss) ("AOCI"):
Prior service cost	0	0	0
Net actuarial loss	6,184	5,637	5,382
Total amount recognized in AOCI	6,184	5,637	5,382
Information for pension plans with an accumulated benefit obligation greater than plan assets:
Projected benefit obligation	16,832	16,847	17,004
Accumulated benefit obligation	16,832	16,847	17,004
Fair value of plan assets	$ 0	$ 0	$ 0