Fair Value Measurements (Financial instruments level within the fair value hierarchy) (Details) - USD ($) $ in Thousands	Sep. 30, 2022	Mar. 31, 2022
Assets:
Short-term investments	$ 2,823,208	$ 2,482,154
Fixed maturities - available for sale	2,552,311	2,821,092
Preferred stock	22,921	26,095
Common stock	40,526	46,212
Derivatives	10,167	7,474
Total	5,449,133	5,383,027
Liabilities:
Derivatives	0	587
Total	0	587
Level 1 [Member]
Assets:
Short-term investments	2,821,751	2,482,154
Fixed maturities - available for sale	24,803	26,914
Preferred stock	22,921	26,095
Common stock	40,526	46,212
Derivatives	1,659	7,474
Total	2,911,660	2,588,849
Liabilities:
Derivatives	0	0
Total	0	0
Level 2 [Member]
Assets:
Short-term investments	1,457	0
Fixed maturities - available for sale	2,527,416	2,794,086
Preferred stock	0	0
Common stock	0	0
Derivatives	8,508	0
Total	2,537,381	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