Comprehensive Income (Reclassification out of AOCI) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net realized and unrealized investment (losses) gain	$ 20,079	$ 7,721	$ 12,649	$ (44,666)	$ (873)	$ (2,183)	$ 4,027	$ 13,451	$ (4,217)	$ 14,422	$ (54,923)
Net investment income	68,511	68,185	34,444	55,967	57,594	55,826	58,505	50,618	227,107	222,543	195,336
Income before federal income tax	159,041	85,257	42,693	15,997	101,293	71,178	90,225	73,694	302,988	336,390	211,721
Income Tax Expense (Benefit)									56,633	64,767	32,782
Net income	$ 127,061	$ 69,875	$ 34,183	$ 15,236	$ 81,859	$ 56,150	$ 72,266	$ 61,348	246,355	271,623	178,939
Loss and loss expense incurred									1,635,823	1,551,491	1,498,134
Policy acquisition costs									366,941	358,069	331,318
Total defined benefit pension and post-retirement plans									3,579	(8,799)	$ (7,226)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Net income									10,594	2,583
Reclassification out of Accumulated Other Comprehensive Income [Member] | Retirement Income Plan [Member]
Income before federal income tax									3,015	2,657
Income Tax Expense (Benefit)									(633)	(558)
Net income									2,382	2,099
Loss and loss expense incurred									647	582
Policy acquisition costs									2,368	2,075
Held-to-maturity Securities [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Net realized and unrealized investment (losses) gain									(16)	(46)
Net investment income									(8)	(12)
Income before federal income tax									(24)	(58)
Income Tax Expense (Benefit)									5	(12)
Net income									(19)	(46)
AOCI, Gain (Loss), Debt Securities, Available-for-sale, without Allowance for Credit Loss, Parent [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Net realized and unrealized investment (losses) gain									5,376	671
Income before federal income tax									5,376	671
Income Tax Expense (Benefit)									(1,129)	(141)
Net income									4,247	530
AOCI, Gain (Loss), Debt Securities, Available-for-sale, with Allowance for Credit Loss, Parent [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Net realized and unrealized investment (losses) gain									5,042	0
Income before federal income tax									5,042	0
Income Tax Expense (Benefit)									(1,058)	0
Net income									$ 3,984	$ 0