Comprehensive Income (Reclassification out of AOCI) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Net realized and unrealized investment gains (losses)	$ 7,721	$ (2,183)	$ (24,296)	$ 15,295
Net investment income earned	68,185	55,826	158,596	164,949
Loss and loss expense incurred	447,802	398,675	1,252,075	1,166,238
Other insurance expenses	89,530	90,234	269,477	261,975
Income before federal income tax	85,257	71,178	143,947	235,097
Total federal income tax expense	(15,382)	(15,028)	(24,653)	(45,333)
Net income	69,875	56,150	119,294	189,764
Reclassification out of AOCI
Net income	(2,064)	2,752	15,320	1,899
Pension Plan [Member] | Reclassification out of AOCI
Loss and loss expense incurred	162	146	486	436
Other insurance expenses	592	519	1,775	1,557
Income before federal income tax	754	665	2,261	1,993
Total federal income tax expense	(158)	(140)	(474)	(419)
Net income	596	525	1,787	1,574
Held-to-maturity Securities [Member] | Reclassification out of AOCI
Net realized and unrealized investment gains (losses)	4	(3)	5	(12)
Net investment income earned	(5)	0	(12)	(21)
Income before federal income tax	(1)	(3)	(7)	(33)
Total federal income tax expense	1	1	2	7
Net income	0	(2)	(5)	(26)
Available for sale securities, excluding credit losses [Member] | Reclassification out of AOCI
Net realized and unrealized investment gains (losses)	(515)	2,821	9,126	444
Income before federal income tax	(515)	2,821	9,126	444
Total federal income tax expense	109	(592)	(1,916)	(93)
Net income	(406)	2,229	7,210	351
Available for sale securities, including credit losses | Reclassification out of AOCI
Net realized and unrealized investment gains (losses)	(2,853)	0	8,010	0
Income before federal income tax	(2,853)	0	8,010	0
Total federal income tax expense	599	0	(1,682)	0
Net income	$ (2,254)	$ 0	$ 6,328	$ 0