Fair Value Measurements (Financial instruments level within the fair value hierarchy) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Mar. 31, 2019
Assets:
Short-term investments	$ 476,776	$ 463,847
Fixed maturities - available for sale	2,440,404	2,209,761
Preferred stock	9,076	8,257
Common stock	19,988	17,379
Derivatives	4,795	1,607
Total	2,951,039	2,700,851
Liabilities:
Derivatives	1,415	0
Total	1,415	0
Level 1 [Member]
Assets:
Short-term investments	476,776	463,599
Fixed maturities - available for sale	7,607	7,327
Preferred stock	9,076	8,257
Common stock	19,988	17,379
Derivatives	4,795	1,468
Total	518,242	498,030
Liabilities:
Derivatives	0	0
Total	0	0
Level 2 [Member]
Assets:
Short-term investments	0	248
Fixed maturities - available for sale	2,432,578	2,202,213
Preferred stock	0	0
Common stock	0	0
Derivatives	0	139
Total	2,432,578	2,202,600
Liabilities:
Derivatives	1,415	0
Total	1,415	0
Level 3 [Member]
Assets:
Short-term investments	0	0
Fixed maturities - available for sale	219	221
Preferred stock	0	0
Common stock	0	0
Derivatives	0	0
Total	219	221
Liabilities:
Derivatives	0	0
Total	$ 0	$ 0