Fair Value Measurements (Financial instruments level within the fair value hierarchy) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Mar. 31, 2019
Assets:
Short-term investments	$ 382,502	$ 463,847
Fixed maturities - available for sale	2,385,120	2,209,761
Preferred stock	9,023	8,257
Common stock	19,207	17,379
Derivatives	5,198	1,607
Total	2,801,050	2,700,851
Liabilities:
Derivatives		0
Total	0	0
Level 1 [Member]
Assets:
Short-term investments	382,252	463,599
Fixed maturities - available for sale	7,502	7,327
Preferred stock	9,023	8,257
Common stock	19,207	17,379
Derivatives	5,174	1,468
Total	423,158	498,030
Liabilities:
Derivatives	0	0
Total	0	0
Level 2 [Member]
Assets:
Short-term investments	250	248
Fixed maturities - available for sale	2,377,399	2,202,213
Preferred stock	0	0
Common stock	0	0
Derivatives	24	139
Total	2,377,673	2,202,600
Liabilities:
Derivatives	2,348	0
Total	2,348	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