Fair Value Measurements (Financial instruments level within the fair value hierarchy) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Mar. 31, 2016
Assets:
Short-term investments	$ 886,301	$ 499,491
Fixed maturities - available for sale	1,659,324	1,466,941
Preferred stock	15,036	18,428
Common stock	26,221	25,169
Derivatives	4,119	3,344
Total	2,591,001	2,013,373
Liabilities:
Guaranteed residual values of TRAC leases
Derivatives	6,785	14,845
Total	6,785	14,845
Level 1 [Member]
Assets:
Short-term investments	886,100	499,288
Fixed maturities - available for sale	98,330	96,328
Preferred stock	15,036	18,428
Common stock	26,221	25,169
Derivatives	4,119	3,344
Total	1,029,806	642,557
Liabilities:
Guaranteed residual values of TRAC leases
Derivatives
Total
Level 2 [Member]
Assets:
Short-term investments	201	203
Fixed maturities - available for sale	1,560,662	1,370,275
Preferred stock
Common stock
Derivatives
Total	1,560,863	1,370,478
Liabilities:
Guaranteed residual values of TRAC leases
Derivatives	6,785	14,845
Total	6,785	14,845
Level 3 [Member]
Assets:
Short-term investments
Fixed maturities - available for sale	332	338
Preferred stock
Common stock
Derivatives
Total	332	338
Liabilities:
Guaranteed residual values of TRAC leases
Derivatives
Total