Fair Value Measurements (Financial instruments level within the fair value hierarchy) (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Mar. 31, 2022
Assets:
Short-term investments	$ 2,193,606	$ 2,482,154
Fixed maturities - available for sale	2,663,528	2,821,092
Preferred stock	22,054	26,095
Common stock	37,619	46,212
Derivatives	10,388	7,474
Total	4,927,195	5,383,027
Liabilities:
Derivatives	0	587
Total	0	587
Level 1 [Member]
Assets:
Short-term investments	2,193,606	2,482,154
Fixed maturities - available for sale	249,161	26,914
Preferred stock	22,054	26,095
Common stock	37,619	46,212
Derivatives	2,182	7,474
Total	2,504,622	2,588,849
Liabilities:
Derivatives	0	0
Total	0	0
Level 2 [Member]
Assets:
Short-term investments	0	0
Fixed maturities - available for sale	2,414,275	2,794,086
Preferred stock	0	0
Common stock	0	0
Derivatives	8,206	0
Total	2,422,481	2,794,086
Liabilities:
Derivatives	0	587
Total	0	587
Level 3 [Member]
Assets:
Short-term investments	0	0
Fixed maturities - available for sale	92	92
Preferred stock	0	0
Common stock	0	0
Derivatives	0	0
Total	92	92
Liabilities:
Derivatives	0	0
Total	$ 0	$ 0