Fair Value Measurements (Financial instruments level within the fair value hierarchy) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Mar. 31, 2017
Assets:
Short-term investments	$ 541,976	$ 521,911
Fixed maturities - available for sale	1,804,501	1,625,845
Preferred stock	14,303	13,489
Common stock	24,964	24,434
Derivatives	3,603	4,260
Total	2,389,347	2,189,939
Liabilities:
Guaranteed residual values of TRAC leases
Derivatives	2,167	4,903
Total	2,167	4,903
Level 1 [Member]
Assets:
Short-term investments	541,776	521,710
Fixed maturities - available for sale	6,850	6,491
Preferred stock	14,303	13,489
Common stock	24,964	24,434
Derivatives	3,603	4,260
Total	591,496	570,384
Liabilities:
Guaranteed residual values of TRAC leases
Derivatives
Total
Level 2 [Member]
Assets:
Short-term investments	200	201
Fixed maturities - available for sale	1,797,323	1,619,024
Preferred stock
Common stock
Derivatives
Total	1,797,523	1,619,225
Liabilities:
Guaranteed residual values of TRAC leases
Derivatives	2,167	4,903
Total	2,167	4,903
Level 3 [Member]
Assets:
Short-term investments
Fixed maturities - available for sale	328	330
Preferred stock
Common stock
Derivatives
Total	328	330
Liabilities:
Guaranteed residual values of TRAC leases
Derivatives
Total