Fair Value Measurements (Financial instruments level within the fair value hierarchy) (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Mar. 31, 2021
Assets:
Short-term investments	$ 2,102,275	$ 839,250
Fixed maturities - available for sale	2,743,151	2,653,539
Preferred stock	28,112	21,677
Common stock	24,179	20,440
Derivatives	6,557	6,601
Total	4,904,274	3,541,507
Liabilities:
Derivatives	2,051	5,141
Total	2,051	5,141
Level 1 [Member]
Assets:
Short-term investments	2,102,275	839,250
Fixed maturities - available for sale	17,103	6,967
Preferred stock	28,112	21,677
Common stock	24,179	20,440
Derivatives	6,557	6,601
Total	2,178,226	894,935
Liabilities:
Derivatives	0	0
Total	0	0
Level 2 [Member]
Assets:
Short-term investments	0	0
Fixed maturities - available for sale	2,725,938	2,646,415
Preferred stock	0	0
Common stock	0	0
Derivatives	0	0
Total	2,725,938	2,646,415
Liabilities:
Derivatives	2,051	5,141
Total	2,051	5,141
Level 3 [Member]
Assets:
Short-term investments	0	0
Fixed maturities - available for sale	110	157
Preferred stock	0	0
Common stock	0	0
Derivatives	0	0
Total	110	157
Liabilities:
Derivatives	0	0
Total	$ 0	$ 0