Fair Value Measurements (Financial Instruments Level within the Fair Value Hierarchy) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Mar. 31, 2017
Assets:
Short-term investments	$ 475,320	$ 521,911
Fixed maturities - available for sale	1,881,137	1,625,845
Preferred stock	10,861	13,489
Common stock	27,862	24,434
Derivatives	4,388	4,260
Total	2,399,568	2,189,939
Liabilities:
Derivatives	460	4,903
Total	460	4,903
Level 1 [Member]
Assets:
Short-term investments	475,320	521,710
Fixed maturities - available for sale	7,567	6,491
Preferred stock	10,861	13,489
Common stock	27,862	24,434
Derivatives	4,388	4,260
Total	525,998	570,384
Liabilities:
Derivatives
Total
Level 2 [Member]
Assets:
Short-term investments		201
Fixed maturities - available for sale	1,873,293	1,619,024
Preferred stock
Common stock
Derivatives
Total	1,873,293	1,619,225
Liabilities:
Derivatives	460	4,903
Total	460	4,903
Level 3 [Member]
Assets:
Short-term investments
Fixed maturities - available for sale	277	330
Preferred stock
Common stock
Derivatives
Total	277	330
Liabilities:
Derivatives
Total