Fair Value Measurements (Financial instruments level within the fair value hierarchy) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Mar. 31, 2018
Assets:
Short-term investments	$ 409,787	$ 475,320
Fixed maturities - available for sale	1,916,049	1,883,384
Preferred stock	10,186	10,861
Common stock	26,852	27,862
Derivatives	4,256	4,825
Total	2,364,919	2,400,005
Liabilities:
Derivatives	338	897
Total	338	897
Level 1 [Member]
Assets:
Short-term investments	409,787	475,320
Fixed maturities - available for sale	7,372	7,567
Preferred stock	10,186	10,861
Common stock	26,852	27,862
Derivatives	3,834	4,388
Total	458,031	525,998
Liabilities:
Derivatives
Total
Level 2 [Member]
Assets:
Short-term investments
Fixed maturities - available for sale	1,906,197	1,873,293
Preferred stock
Common stock
Derivatives	422	437
Total	1,906,619	1,873,730
Liabilities:
Derivatives	338	897
Total	338	897
Level 3 [Member]
Assets:
Short-term investments
Fixed maturities - available for sale	269	277
Preferred stock
Common stock
Derivatives
Total	269	277
Liabilities:
Derivatives
Total