Investments (Securities Available for Sale) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Debt securities:
Totals before allowance for credit losses	$ 3,156,486	$ 3,206,120
Gross Unrealized Gains	274,722	153,424
Gross Unrealized Losses	(20,552)	(2,599)
Fair Value	3,410,656	3,356,945
Allowance for Credit Losses	0		$ 0	$ 0	$ 0
Fair Value
Less than 12 Months	195,431	40,550
12 Months or Greater	28,961	29,292
Total	224,392	69,842
Unrealized Losses
Less than 12 Months	(17,828)	(108)
12 Months or Greater	(2,724)	(2,491)
Total	(20,552)	(2,599)
States and political subdivisions
Debt securities:
Totals before allowance for credit losses	89,168	98,037
Gross Unrealized Gains	8,384	4,495
Gross Unrealized Losses	(2)	(3)
Fair Value	97,550	102,529
Allowance for Credit Losses	0
Fair Value
Less than 12 Months	558	470
12 Months or Greater	0	0
Total	558	470
Unrealized Losses
Less than 12 Months	(2)	(3)
12 Months or Greater	0	0
Total	(2)	(3)
Foreign governments
Debt securities:
Totals before allowance for credit losses	9,990	9,983
Gross Unrealized Gains	235	203
Gross Unrealized Losses	0	0
Fair Value	10,225	10,186
Allowance for Credit Losses	0
Public utilities
Debt securities:
Totals before allowance for credit losses	62,413	67,895
Gross Unrealized Gains	5,194	3,476
Gross Unrealized Losses	0	0
Fair Value	67,607	71,371
Allowance for Credit Losses	0
Fair Value
Less than 12 Months	0	0
12 Months or Greater	0	0
Total	0	0
Unrealized Losses
Less than 12 Months	0	0
12 Months or Greater	0	0
Total	0	0
Corporate
Debt securities:
Totals before allowance for credit losses	2,895,799	2,921,431
Gross Unrealized Gains	256,716	141,705
Gross Unrealized Losses	(20,349)	(2,479)
Fair Value	3,132,166	3,060,657
Allowance for Credit Losses	0
Fair Value
Less than 12 Months	179,742	40,080
12 Months or Greater	28,389	28,582
Total	208,131	68,662
Unrealized Losses
Less than 12 Months	(17,784)	(105)
12 Months or Greater	(2,565)	(2,374)
Total	(20,349)	(2,479)
Commercial mortgage-backed
Debt securities:
Totals before allowance for credit losses	27,124	28,871
Gross Unrealized Gains	1,187	1,071
Gross Unrealized Losses	0	0
Fair Value	28,311	29,942
Allowance for Credit Losses	0
Fair Value
Less than 12 Months	0
12 Months or Greater	0
Total	0
Unrealized Losses
Less than 12 Months	0
12 Months or Greater	0
Total	0
Residential mortgage-backed
Debt securities:
Totals before allowance for credit losses	10,653	12,815
Gross Unrealized Gains	1,247	1,077
Gross Unrealized Losses	(159)	(117)
Fair Value	11,741	13,775
Allowance for Credit Losses	0
Fair Value
Less than 12 Months	0	0
12 Months or Greater	572	710
Total	572	710
Unrealized Losses
Less than 12 Months	0	0
12 Months or Greater	(159)	(117)
Total	(159)	(117)
Asset-backed
Debt securities:
Totals before allowance for credit losses	61,339	67,088
Gross Unrealized Gains	1,759	1,397
Gross Unrealized Losses	(42)	0
Fair Value	63,056	$ 68,485
Allowance for Credit Losses	0
Fair Value
Less than 12 Months	15,131
12 Months or Greater	0
Total	15,131
Unrealized Losses
Less than 12 Months	(42)
12 Months or Greater	0
Total	$ (42)