Fair Value Measurements (Financial instruments level within the fair value hierarchy) (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Mar. 31, 2015
Assets:
Short-term investments	$ 825,071	$ 460,762
Fixed maturities - available for sale	1,318,081	1,262,012
Preferred stock	18,098	18,296
Common stock	24,601	24,654
Derivatives	3,463	4,876
Total	$ 2,189,314	$ 1,770,600
Liabilities:
Guaranteed residual values of TRAC leases
Derivatives	$ 19,958	$ 24,484
Total	19,958	24,484
Level 1 [Member]
Assets:
Short-term investments	825,071	460,762
Fixed maturities - available for sale	1,041,024	967,986
Preferred stock	18,098	18,296
Common stock	24,601	24,654
Derivatives	3,463	4,876
Total	$ 1,912,257	$ 1,476,574
Liabilities:
Guaranteed residual values of TRAC leases
Derivatives
Total
Level 2 [Member]
Assets:
Short-term investments
Fixed maturities - available for sale	$ 276,177	$ 293,022
Preferred stock
Common stock
Derivatives
Total	$ 276,177	$ 293,022
Liabilities:
Guaranteed residual values of TRAC leases
Derivatives	$ 19,958	$ 24,484
Total	$ 19,958	$ 24,484
Level 3 [Member]
Assets:
Short-term investments
Fixed maturities - available for sale	$ 880	$ 1,004
Preferred stock
Common stock
Derivatives
Total	$ 880	$ 1,004
Liabilities:
Guaranteed residual values of TRAC leases
Derivatives
Total