Fair Value Measurements (Financial instruments level within the fair value hierarchy) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Mar. 31, 2018
Assets:
Short-term investments	$ 670,795	$ 475,320
Fixed maturities - available for sale	2,100,229	1,881,137
Preferred stock	8,815	10,861
Common stock	18,298	27,862
Derivatives	4,783	4,825
Total	2,802,920	2,400,005
Liabilities:
Derivatives		897
Total		897
Level 1 [Member]
Assets:
Short-term investments	670,795	475,320
Fixed maturities - available for sale	7,190	7,567
Preferred stock	8,815	10,861
Common stock	18,298	27,862
Derivatives	4,543	4,388
Total	709,641	525,998
Liabilities:
Derivatives
Total
Level 2 [Member]
Assets:
Short-term investments
Fixed maturities - available for sale	2,092,818	1,873,293
Preferred stock
Common stock
Derivatives	240	437
Total	2,093,058	1,873,730
Liabilities:
Derivatives		897
Total		897
Level 3 [Member]
Assets:
Short-term investments
Fixed maturities - available for sale	221	277
Preferred stock
Common stock
Derivatives
Total	221	277
Liabilities:
Derivatives
Total