Fair Value Measurements (Financial instruments level within the fair value hierarchy) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Mar. 31, 2021
Assets:
Short-term investments	$ 1,311,376	$ 839,250
Fixed maturities - available for sale	2,703,053	2,653,539
Preferred stock	29,408	21,677
Common stock	22,672	20,440
Derivatives	6,789	6,601
Total	4,073,298	3,541,507
Liabilities:
Derivatives	4,222	5,141
Total	4,222	5,141
Level 1 [Member]
Assets:
Short-term investments	1,311,376	839,250
Fixed maturities - available for sale	7,185	6,967
Preferred stock	29,408	21,677
Common stock	22,672	20,440
Derivatives	6,789	6,601
Total	1,377,430	894,935
Liabilities:
Derivatives	0	0
Total	0	0
Level 2 [Member]
Assets:
Short-term investments	0	0
Fixed maturities - available for sale	2,695,712	2,646,415
Preferred stock	0	0
Common stock	0	0
Derivatives	0	0
Total	2,695,712	2,646,415
Liabilities:
Derivatives	4,222	5,141
Total	4,222	5,141
Level 3 [Member]
Assets:
Short-term investments	0	0
Fixed maturities - available for sale	156	157
Preferred stock	0	0
Common stock	0	0
Derivatives	0	0
Total	156	157
Liabilities:
Derivatives	0	0
Total	$ 0	$ 0