Fair Value Measurements (Financial instruments level within the fair value hierarchy) (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Mar. 31, 2022
Assets:
Short-term investments	$ 2,845,484	$ 2,482,154
Fixed maturities - available for sale	2,598,199	2,821,092
Preferred stock	25,950	26,095
Common stock	45,837	46,212
Derivatives	7,111	7,474
Total	5,522,581	5,383,027
Liabilities:
Derivatives	0	587
Total	0	587
Level 1 [Member]
Assets:
Short-term investments	2,845,484	2,482,154
Fixed maturities - available for sale	25,456	26,914
Preferred stock	25,950	26,095
Common stock	45,837	46,212
Derivatives	6,962	7,474
Total	2,949,689	2,588,849
Liabilities:
Derivatives	0	0
Total	0	0
Level 2 [Member]
Assets:
Short-term investments	0	0
Fixed maturities - available for sale	2,572,651	2,794,086
Preferred stock	0	0
Common stock	0	0
Derivatives	149	0
Total	2,572,800	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