Employee Benefits (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Interest percentage added to the five-year trailing average of five-year Treasury Securities	1.50%
Interest percentage accrued on employee account balances, Minimum	3.00%
Interest percentage accrued on employee account balances, Maximum	5.00%
Quarterly variable interest percentage accrued on employee account balances, minimum	3.00%
Quarterly variable interest percentage accrued on employee account balances, maximum	3.24%
Change in projected benefit obligation [Roll Forward]
Projected benefit obligation at beginning of year	$ 30,897,000	[1],[2]	$ 34,964,000
Interest cost	973,000		1,153,000
Actuarial loss (gain)	(1,417,000)		223,000
Benefits paid	(6,033,000)		(5,443,000)
Projected benefit obligation at end of year	24,547,000	[1],[2]	30,897,000	[1],[2]	$ 34,964,000
Change in plan assets [Roll Forward]
Plan assets at fair value at beginning of year	40,419,000		41,769,000
Actual return on plan assets	(804,000)		4,093,000
Benefits paid	(6,033,000)		(5,443,000)
Plan assets at fair value at end of year	33,582,000		40,419,000		41,769,000
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Funded status of the plan	9,035,000		9,522,000
Components of net periodic benefit costs [Abstract]
Interest cost	973,000		1,153,000
Expected return on plan assets	(2,065,000)		(2,041,000)
Other	509,000		184,000
Net periodic benefit cost (credit)	$ (583,000)		$ (704,000)
Weighted-average assumptions [Abstract]
Discount rate (in hundredths)	4.10%		3.30%
Expected return on plan assets (in hundredths)	5.50%		5.50%
Estimated Future Benefit Payments [Abstract]
2019	$ 3,562,000
2020	2,257,000
2021	2,194,000
2022	2,299,000
2023	2,495,000
Thereafter	19,175,000
Total estimated future benefit payments	31,982,000
Defined Benefit Plan, Information about Plan Assets [Abstract]
Maximum tax deductible contributions to defined benefit plan	$ 6,600,000
Thrift Plan - Defined Contribution [Abstract]
Employee base compensation company match employee contributions (in hundredths)	6.00%
Employer matching rate for employee contributions minimum (in hundredths)	50.00%
Minimum years of service for employees to obtain maximum employer matching (in years)	4 years
Employer matching rate for employee contributions maximum (in hundredths)	200.00%
Maximum years of service for employees to obtain minimum employer matching (in years)	15 years
Non-elective annual contribution for qualified employees	$ 750
Annual base employee compensation to qualify for non-elective employer contributions maximum	$ 40,000
Vesting period for employer contributions (in years)	5 years
Thrift Plan expenses	$ 25,100,000		$ 22,800,000		$ 22,400,000
Equity Securities [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	60.00%
Bonds [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	40.00%

[1] Projected benefit obligation equals accumulated benefit obligation.

[2] Projected benefit obligation is based on January 1 measurement date.