Investments (Securities Available for Sale) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt securities:
Total	$ 3,265,807	$ 3,008,624
Gross Unrealized Gains	149,111	17,678
Gross Unrealized Losses	(5,452)	(80,243)
Fair Value	3,409,466	2,946,059
Fair Value
Less than 12 Months	15,334	1,508,422
12 Months or Greater	28,561	443,676
Total	43,895	1,952,098
Unrealized Losses
Less than 12 Months	(141)	(45,096)
12 Months or Greater	(5,311)	(35,147)
Total	(5,452)	(80,243)
States and political subdivisions
Debt securities:
Total	100,595	570
Gross Unrealized Gains	6,178	0
Gross Unrealized Losses	(2)	(4)
Fair Value	106,771	566
Fair Value
Less than 12 Months	474	566
12 Months or Greater	0	0
Total	474	566
Unrealized Losses
Less than 12 Months	(2)	(4)
12 Months or Greater	0	0
Total	(2)	(4)
Foreign governments
Debt securities:
Total	9,981	9,974
Gross Unrealized Gains	244	30
Gross Unrealized Losses	0	0
Fair Value	10,225	10,004
Public utilities
Debt securities:
Total	76,916	82,943
Gross Unrealized Gains	3,914	1,045
Gross Unrealized Losses	0	(517)
Fair Value	80,830	83,471
Fair Value
Less than 12 Months		38,903
12 Months or Greater		0
Total		38,903
Unrealized Losses
Less than 12 Months		(517)
12 Months or Greater		0
Total		(517)
Corporate
Debt securities:
Total	2,960,272	2,893,221
Gross Unrealized Gains	133,840	15,473
Gross Unrealized Losses	(5,343)	(79,638)
Fair Value	3,088,769	2,829,056
Fair Value
Less than 12 Months	14,860	1,468,953
12 Months or Greater	27,763	442,798
Total	42,623	1,911,751
Unrealized Losses
Less than 12 Months	(139)	(44,575)
12 Months or Greater	(5,204)	(35,063)
Total	(5,343)	(79,638)
Commercial mortgage-backed
Debt securities:
Total	28,902
Gross Unrealized Gains	1,375
Gross Unrealized Losses	0
Fair Value	30,277
Residential mortgage-backed
Debt securities:
Total	13,621	15,947
Gross Unrealized Gains	1,213	937
Gross Unrealized Losses	(107)	(84)
Fair Value	14,727	16,800
Fair Value
Less than 12 Months	0	0
12 Months or Greater	798	878
Total	798	878
Unrealized Losses
Less than 12 Months	0	0
12 Months or Greater	(107)	(84)
Total	(107)	(84)
Asset-backed
Debt securities:
Total	75,520	5,969
Gross Unrealized Gains	2,347	193
Gross Unrealized Losses	0	0
Fair Value	$ 77,867	$ 6,162

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