Fair Value Measurements (Financial Instruments Level within the Fair Value Hierarchy) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Mar. 31, 2016
Assets:
Short-term investments	$ 521,911	$ 499,491
Fixed maturities - available for sale	1,625,845	1,448,941
Preferred stock	13,489	18,428
Common stock	24,434	23,420
Derivatives	4,260	3,344
Total	2,189,939	1,993,624
Liabilities:
Guaranteed residual values of TRAC leases
Derivatives	4,903	14,845
Total	4,903	14,845
Level 1 [Member]
Assets:
Short-term investments	521,710	499,491
Fixed maturities - available for sale	6,491	96,328
Preferred stock	13,489	18,428
Common stock	24,434	23,420
Derivatives	4,260	3,344
Total	570,384	641,011
Liabilities:
Guaranteed residual values of TRAC leases
Derivatives
Total
Level 2 [Member]
Assets:
Short-term investments	201
Fixed maturities - available for sale	1,619,024	1,352,275
Preferred stock
Common stock
Derivatives
Total	1,619,225	1,352,275
Liabilities:
Guaranteed residual values of TRAC leases
Derivatives	4,903	14,845
Total	4,903	14,845
Level 3 [Member]
Assets:
Short-term investments
Fixed maturities - available for sale	330	338
Preferred stock
Common stock
Derivatives
Total	330	338
Liabilities:
Guaranteed residual values of TRAC leases
Derivatives
Total