Employee Benefits (Obligations and Funded Status of the Plan) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retirement Income Plan
Change in projected benefit obligation:
Benefit obligation at end of prior year	$ 337,768	$ 325,706
Service cost	8,156	7,705	$ 8,530
Interest cost	12,196	12,161	13,477
Actuarial (gain) loss	20,829	7,988
Benefits paid	(16,960)	(15,792)
Benefit obligation at end of year	361,989	337,768	325,706
Change in plan assets:
Fair value of plan assets at end of prior year	269,766	260,035
Actual return (loss) on plan assets	44,283	18,223
Employer contribution	7,300	7,300
Benefits paid	(16,960)	(15,792)
Fair value of plan assets at end of year	304,389	269,766	260,035
Funded status at end of year	(57,600)	(68,002)
Other Pension Plans
Change in projected benefit obligation:
Benefit obligation at end of prior year	27,462	26,958
Service cost	362	296	262
Interest cost	863	878	1,018
Actuarial (gain) loss	2,217	1,267
Benefits paid	(2,512)	(1,937)
Benefit obligation at end of year	28,392	27,462	26,958
Change in plan assets:
Fair value of plan assets at end of prior year	0	0
Actual return (loss) on plan assets	0	0
Employer contribution	2,512	1,937
Benefits paid	(2,512)	(1,937)
Fair value of plan assets at end of year	0	0	$ 0
Funded status at end of year	$ (28,392)	$ (27,462)