Fair Value Measurements (Financial instruments level within the fair value hierarchy) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 31, 2020
Assets:
Short-term investments	$ 629,550	$ 369,279
Fixed maturities - available for sale	2,370,821	2,466,048
Preferred stock	6,106	6,675
Common stock	16,595	20,015
Derivatives	1,763	5,944
Total	3,024,835	2,867,961
Liabilities:
Derivatives	8,170	8,214
Total	8,170	8,214
Level 1 [Member]
Assets:
Short-term investments	629,265	368,968
Fixed maturities - available for sale	7,614	7,156
Preferred stock	6,106	6,675
Common stock	16,595	20,015
Derivatives	1,763	5,944
Total	661,343	408,758
Liabilities:
Derivatives	0	0
Total	0	0
Level 2 [Member]
Assets:
Short-term investments	285	311
Fixed maturities - available for sale	2,363,047	2,458,731
Preferred stock	0	0
Common stock	0	0
Derivatives	0	0
Total	2,363,332	2,459,042
Liabilities:
Derivatives	8,170	8,214
Total	8,170	8,214
Level 3 [Member]
Assets:
Short-term investments	0	0
Fixed maturities - available for sale	160	161
Preferred stock	0	0
Common stock	0	0
Derivatives	0	0
Total	160	161
Liabilities:
Derivatives	0	0
Total	$ 0	$ 0