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Investments (Investments Held in a Loss Position) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Fair value
Fair Value	$ 1,541,516	$ 212,934
Less than 12 Months, Fair Value	1,486,475	191,081
More than 12 Months, Fair Value	55,041	21,853
Unrealized Loss
Unrealized Loss	14,318	3,428
Less than 12 Months, Unrealized Loss	12,712	3,099
More than 12 Months, Unrealized Loss	1,606	329
U.S. Treasury obligations
Fair value
Fair Value	162,063	14,390
Less than 12 Months, Fair Value	157,445	14,390
More than 12 Months, Fair Value	4,618	0
Unrealized Loss
Unrealized Loss	672	23
Less than 12 Months, Unrealized Loss	582	23
More than 12 Months, Unrealized Loss	90	0
U.S. Government-sponsored enterprise obligations
Fair value
Fair Value	14,422
Less than 12 Months, Fair Value	14,422
More than 12 Months, Fair Value	0
Unrealized Loss
Unrealized Loss	98
Less than 12 Months, Unrealized Loss	98
More than 12 Months, Unrealized Loss	0
State and municipal bonds
Fair value
Fair Value	194,740	6,416
Less than 12 Months, Fair Value	192,610	6,416
More than 12 Months, Fair Value	2,130	0
Unrealized Loss
Unrealized Loss	1,559	39
Less than 12 Months, Unrealized Loss	1,467	39
More than 12 Months, Unrealized Loss	92	0
Corporate debt
Fair value
Fair Value	637,694	94,695
Less than 12 Months, Fair Value	607,003	79,436
More than 12 Months, Fair Value	30,691	15,259
Unrealized Loss
Unrealized Loss	6,923	1,302
Less than 12 Months, Unrealized Loss	6,134	1,020
More than 12 Months, Unrealized Loss	789	282
Residential mortgage-backed securities
Fair value
Fair Value	284,636	34,928
Less than 12 Months, Fair Value	276,253	34,509
More than 12 Months, Fair Value	8,383	419
Unrealized Loss
Unrealized Loss	2,972	382
Less than 12 Months, Unrealized Loss	2,732	381
More than 12 Months, Unrealized Loss	240	1
Agency commercial mortgage-backed securities
Fair value
Fair Value	4,040
Less than 12 Months, Fair Value	4,040
More than 12 Months, Fair Value	0
Unrealized Loss
Unrealized Loss	49
Less than 12 Months, Unrealized Loss	49
More than 12 Months, Unrealized Loss	0
Other commercial mortgage-backed securities
Fair value
Fair Value	77,254	18,766
Less than 12 Months, Fair Value	72,541	18,480
More than 12 Months, Fair Value	4,713	286
Unrealized Loss
Unrealized Loss	1,116	940
Less than 12 Months, Unrealized Loss	783	935
More than 12 Months, Unrealized Loss	333	5
Other asset-backed securities
Fair value
Fair Value	166,667	43,739
Less than 12 Months, Fair Value	162,161	37,850
More than 12 Months, Fair Value	4,506	5,889
Unrealized Loss
Unrealized Loss	929	742
Less than 12 Months, Unrealized Loss	867	701
More than 12 Months, Unrealized Loss	$ 62	$ 41

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