Fair Value Measurements (Financial instruments level within the fair value hierarchy) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Mar. 31, 2019
Assets:
Short-term investments	$ 365,801	$ 463,847
Fixed maturities - available for sale	2,308,689	2,209,761
Preferred stock	8,861	8,257
Common stock	19,035	17,379
Derivatives	4,555	1,607
Total	2,706,941	2,700,851
Liabilities:
Derivatives	1,181	0
Total	1,181	0
Level 1 [Member]
Assets:
Short-term investments	365,552	463,599
Fixed maturities - available for sale	7,333	7,327
Preferred stock	8,861	8,257
Common stock	19,035	17,379
Derivatives	4,491	1,468
Total	405,272	498,030
Liabilities:
Derivatives	0	0
Total	0	0
Level 2 [Member]
Assets:
Short-term investments	249	248
Fixed maturities - available for sale	2,301,135	2,202,213
Preferred stock	0	0
Common stock	0	0
Derivatives	64	139
Total	2,301,448	2,202,600
Liabilities:
Derivatives	1,181	0
Total	1,181	0
Level 3 [Member]
Assets:
Short-term investments	0	0
Fixed maturities - available for sale	221	221
Preferred stock	0	0
Common stock	0	0
Derivatives	0	0
Total	221	221
Liabilities:
Derivatives	0	0
Total	$ 0	$ 0