Fair Value Measurements (Financial instruments level within the fair value hierarchy) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Mar. 31, 2020
Assets:
Short-term investments	$ 1,128,967	$ 369,279
Fixed maturities - available for sale	2,565,293	2,466,048
Preferred stock	22,215	6,675
Common stock	19,445	20,015
Derivatives	5,797	5,944
Total	3,741,717	2,867,961
Liabilities:
Derivatives	6,158	8,214
Total	6,158	8,214
Level 1 [Member]
Assets:
Short-term investments	1,128,667	368,968
Fixed maturities - available for sale	7,602	7,156
Preferred stock	22,215	6,675
Common stock	19,445	20,015
Derivatives	5,797	5,944
Total	1,183,726	408,758
Liabilities:
Derivatives	0	0
Total	0	0
Level 2 [Member]
Assets:
Short-term investments	300	311
Fixed maturities - available for sale	2,557,532	2,458,731
Preferred stock	0	0
Common stock	0	0
Derivatives	0	0
Total	2,557,832	2,459,042
Liabilities:
Derivatives	6,158	8,214
Total	6,158	8,214
Level 3 [Member]
Assets:
Short-term investments	0	0
Fixed maturities - available for sale	159	161
Preferred stock	0	0
Common stock	0	0
Derivatives	0	0
Total	159	161
Liabilities:
Derivatives	0	0
Total	$ 0	$ 0