Retirement Plans (Components of Net Periodic Benefit Cost) (Details) - Pension Plan [Member] - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Interest cost		$ 2,828	$ 3,376	$ 8,484	$ 10,129
Expected return on plan assets		(5,477)	(5,278)	(16,430)	(15,835)
Amortization of unrecognized net actuarial loss		704	644	2,112	1,931
Total net periodic pension cost (benefit)	[1]	$ (1,945)	$ (1,258)	$ (5,834)	$ (3,775)

[1] The components of net periodic pension cost (benefit) are included within "Loss and loss expense incurred" and "Other insurance expenses" on the Consolidated Statements of Income.