Fair Value Measurements (Financial instruments level within the fair value hierarchy) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Mar. 31, 2016
Assets:
Short-term investments	$ 850,260	$ 499,491
Fixed maturities - available for sale	1,588,995	1,466,941
Preferred stock	16,000	18,428
Common stock	27,425	25,169
Derivatives	3,534	3,344
Total	2,486,214	2,013,373
Liabilities:
Guaranteed residual values of TRAC leases
Derivatives	9,654	14,845
Total	9,654	14,845
Level 1 [Member]
Assets:
Short-term investments	850,058	499,288
Fixed maturities - available for sale	98,825	96,328
Preferred stock	16,000	18,428
Common stock	27,425	25,169
Derivatives	3,534	3,344
Total	995,842	642,557
Liabilities:
Guaranteed residual values of TRAC leases
Derivatives
Total
Level 2 [Member]
Assets:
Short-term investments	202	203
Fixed maturities - available for sale	1,489,836	1,370,275
Preferred stock
Common stock
Derivatives
Total	1,490,038	1,370,478
Liabilities:
Guaranteed residual values of TRAC leases
Derivatives	9,654	14,845
Total	9,654	14,845
Level 3 [Member]
Assets:
Short-term investments
Fixed maturities - available for sale	334	338
Preferred stock
Common stock
Derivatives
Total	334	338
Liabilities:
Guaranteed residual values of TRAC leases
Derivatives
Total