Fair Value Measurements (Financial instruments level within the fair value hierarchy) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Mar. 31, 2021
Assets:
Short-term investments	$ 2,215,846	$ 839,250
Fixed maturities - available for sale	2,790,243	2,653,539
Preferred stock	30,176	21,677
Common stock	24,728	20,440
Derivatives	5,754	6,601
Total	5,066,747	3,541,507
Liabilities:
Derivatives	3,292	5,141
Total	3,292	5,141
Level 1 [Member]
Assets:
Short-term investments	2,215,846	839,250
Fixed maturities - available for sale	6,721	6,967
Preferred stock	30,176	21,677
Common stock	24,728	20,440
Derivatives	5,754	6,601
Total	2,283,225	894,935
Liabilities:
Derivatives	0	0
Total	0	0
Level 2 [Member]
Assets:
Short-term investments	0	0
Fixed maturities - available for sale	2,783,407	2,646,415
Preferred stock	0	0
Common stock	0	0
Derivatives	0	0
Total	2,783,407	2,646,415
Liabilities:
Derivatives	3,292	5,141
Total	3,292	5,141
Level 3 [Member]
Assets:
Short-term investments	0	0
Fixed maturities - available for sale	115	157
Preferred stock	0	0
Common stock	0	0
Derivatives	0	0
Total	115	157
Liabilities:
Derivatives	0	0
Total	$ 0	$ 0