Fair Value Measurements (Financial instruments level within the fair value hierarchy) (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Mar. 31, 2016
Assets:
Short-term investments	$ 556,773	$ 499,491
Fixed maturities - available for sale	1,613,457	1,466,941
Preferred stock	16,315	18,428
Common stock	26,387	25,169
Derivatives	3,353	3,344
Total	2,216,285	2,013,373
Liabilities:
Guaranteed residual values of TRAC leases
Derivatives	12,766	14,845
Total	12,766	14,845
Level 1 [Member]
Assets:
Short-term investments	556,571	499,288
Fixed maturities - available for sale	99,274	96,328
Preferred stock	16,315	18,428
Common stock	26,387	25,169
Derivatives	3,353	3,344
Total	701,900	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	1,513,847	1,370,275
Preferred stock
Common stock
Derivatives
Total	1,514,049	1,370,478
Liabilities:
Guaranteed residual values of TRAC leases
Derivatives	12,766	14,845
Total	12,766	14,845
Level 3 [Member]
Assets:
Short-term investments
Fixed maturities - available for sale	336	338
Preferred stock
Common stock
Derivatives
Total	336	338
Liabilities:
Guaranteed residual values of TRAC leases
Derivatives
Total