Fair Value Measurements (Financial instruments level within the fair value hierarchy) (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Mar. 31, 2023
Assets:
Short-term investments	$ 2,055,145	$ 1,809,441
Fixed maturities - available for sale	2,407,226	2,709,037
Preferred stock	20,556	21,982
Common stock	41,730	39,375
Derivatives	16,793	9,606
Total	4,541,450	4,589,441
Liabilities:
Derivatives	0	0
Level 1 [Member]
Assets:
Short-term investments	2,053,510	1,808,797
Fixed maturities - available for sale	24,549	251,832
Preferred stock	20,556	21,982
Common stock	41,730	39,375
Derivatives	7,539	4,295
Total	2,147,884	2,126,281
Level 2 [Member]
Assets:
Short-term investments	1,635	644
Fixed maturities - available for sale	2,382,618	2,457,146
Preferred stock	0	0
Common stock	0	0
Derivatives	9,254	5,311
Total	2,393,507	2,463,101
Level 3 [Member]
Assets:
Short-term investments	0	0
Fixed maturities - available for sale	59	59
Preferred stock	0	0
Common stock	0	0
Derivatives	0	0
Total	$ 59	$ 59