Investments (Securities Available for Sale) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Debt securities:
Total	$ 3,166,206	$ 3,206,120
Gross Unrealized Gains	234,339	153,424
Gross Unrealized Losses	(27,432)	(2,599)
Fair Value	3,373,113	3,356,945
Allowance for Credit Losses	0		$ 0	$ 0	$ 0
Fair Value
Less than 12 Months	249,437	40,550
12 Months or Greater	29,307	29,292
Total	278,744	69,842
Unrealized Losses
Less than 12 Months	(24,566)	(108)
12 Months or Greater	(2,866)	(2,491)
Total	(27,432)	(2,599)
States and political subdivisions
Debt securities:
Total	89,288	98,037
Gross Unrealized Gains	7,192	4,495
Gross Unrealized Losses	(5)	(3)
Fair Value	96,475	102,529
Allowance for Credit Losses	0
Fair Value
Less than 12 Months	558	470
12 Months or Greater	464	0
Total	1,022	470
Unrealized Losses
Less than 12 Months	(4)	(3)
12 Months or Greater	(1)	0
Total	(5)	(3)
Foreign governments
Debt securities:
Total	9,988	9,983
Gross Unrealized Gains	312	203
Gross Unrealized Losses	0	0
Fair Value	10,300	10,186
Allowance for Credit Losses	0
Public utilities
Debt securities:
Total	64,407	67,895
Gross Unrealized Gains	4,915	3,476
Gross Unrealized Losses	0	0
Fair Value	69,322	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:
Total	2,905,567	2,921,431
Gross Unrealized Gains	218,647	141,705
Gross Unrealized Losses	(26,288)	(2,479)
Fair Value	3,097,926	3,060,657
Allowance for Credit Losses	0
Fair Value
Less than 12 Months	217,451	40,080
12 Months or Greater	28,249	28,582
Total	245,700	68,662
Unrealized Losses
Less than 12 Months	(23,582)	(105)
12 Months or Greater	(2,706)	(2,374)
Total	(26,288)	(2,479)
Commercial mortgage-backed
Debt securities:
Total	27,137	28,871
Gross Unrealized Gains	613	1,071
Gross Unrealized Losses	(319)	0
Fair Value	27,431	29,942
Allowance for Credit Losses	0
Fair Value
Less than 12 Months	8,772
12 Months or Greater	0
Total	8,772
Unrealized Losses
Less than 12 Months	(319)
12 Months or Greater	0
Total	(319)
Residential mortgage-backed
Debt securities:
Total	11,381	12,815
Gross Unrealized Gains	1,319	1,077
Gross Unrealized Losses	(159)	(117)
Fair Value	12,541	13,775
Allowance for Credit Losses	0
Fair Value
Less than 12 Months	0	0
12 Months or Greater	594	710
Total	594	710
Unrealized Losses
Less than 12 Months	0	0
12 Months or Greater	(159)	(117)
Total	(159)	(117)
Asset-backed
Debt securities:
Total	58,438	67,088
Gross Unrealized Gains	1,341	1,397
Gross Unrealized Losses	(661)	0
Fair Value	59,118	$ 68,485
Allowance for Credit Losses	0
Fair Value
Less than 12 Months	22,656
12 Months or Greater	0
Total	22,656
Unrealized Losses
Less than 12 Months	(661)
12 Months or Greater	0
Total	$ (661)