Fair Value Measurements (Financial instruments level within the fair value hierarchy) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Mar. 31, 2018
Assets:
Short-term investments	$ 401,666	$ 475,320
Fixed maturities - available for sale	1,979,164	1,881,137
Preferred stock	9,565	10,861
Common stock	26,673	27,862
Derivatives	3,855	4,825
Total	2,420,923	2,400,005
Liabilities:
Derivatives		897
Total		897
Level 1 [Member]
Assets:
Short-term investments	401,666	475,320
Fixed maturities - available for sale	7,298	7,567
Preferred stock	9,565	10,861
Common stock	26,673	27,862
Derivatives	3,496	4,388
Total	448,698	525,998
Liabilities:
Derivatives
Total
Level 2 [Member]
Assets:
Short-term investments
Fixed maturities - available for sale	1,971,616	1,873,293
Preferred stock
Common stock
Derivatives	359	437
Total	1,971,975	1,873,730
Liabilities:
Derivatives		897
Total		897
Level 3 [Member]
Assets:
Short-term investments
Fixed maturities - available for sale	250	277
Preferred stock
Common stock
Derivatives
Total	250	277
Liabilities:
Derivatives
Total