Investments - Unrealized Losses (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Cost or amortized Cost
Total Cost or Amortized Cost	$ 2,952,414	$ 2,694,739
Fair value
Total Fair Value	2,622,851	2,414,517
Unrealized Loss
Total Unrealized Loss	$ (329,563)	$ (280,222)
Percent to Total	100.00%
NAIC Rating 1 | AAA/AA/A | Aaa/Aa/A
Cost or amortized Cost
Total Cost or Amortized Cost	$ 2,440,094
Fair value
Total Fair Value	2,160,142
Unrealized Loss
Total Unrealized Loss	$ (279,952)
Percent to Total	85.00%
NAIC Rating 2 | BBB | Baa
Cost or amortized Cost
Total Cost or Amortized Cost	$ 431,695
Fair value
Total Fair Value	388,396
Unrealized Loss
Total Unrealized Loss	$ (43,299)
Percent to Total	13.10%
NAIC Rating 3 | BB | Ba
Cost or amortized Cost
Total Cost or Amortized Cost	$ 48,627
Fair value
Total Fair Value	44,282
Unrealized Loss
Total Unrealized Loss	$ (4,345)
Percent to Total	1.30%
NAIC Rating 4 | B | B
Cost or amortized Cost
Total Cost or Amortized Cost	$ 29,382
Fair value
Total Fair Value	28,045
Unrealized Loss
Total Unrealized Loss	$ (1,337)
Percent to Total	0.40%
NAIC Rating 5 | CCC | Caa
Cost or amortized Cost
Total Cost or Amortized Cost	$ 2,616
Fair value
Total Fair Value	1,986
Unrealized Loss
Total Unrealized Loss	$ (630)
Percent to Total	0.20%
NAIC Rating 6 | CC Or Lower | Ca or lower
Unrealized Loss
Percent to Total	0.00%