Fair Value Measurements (Financial instruments level within the fair value hierarchy) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Mar. 31, 2017
Assets:
Short-term investments	$ 546,743	$ 521,911
Fixed maturities - available for sale	1,724,331	1,625,845
Preferred stock	14,090	13,489
Common stock	24,610	24,434
Derivatives	4,288	4,260
Total	2,314,062	2,189,939
Liabilities:
Guaranteed residual values of TRAC leases
Derivatives	3,353	4,903
Total	3,353	4,903
Level 1 [Member]
Assets:
Short-term investments	546,542	521,710
Fixed maturities - available for sale	6,506	6,491
Preferred stock	14,090	13,489
Common stock	24,610	24,434
Derivatives	4,288	4,260
Total	596,036	570,384
Liabilities:
Guaranteed residual values of TRAC leases
Derivatives
Total
Level 2 [Member]
Assets:
Short-term investments	201	201
Fixed maturities - available for sale	1,717,496	1,619,024
Preferred stock
Common stock
Derivatives
Total	1,717,697	1,619,225
Liabilities:
Guaranteed residual values of TRAC leases
Derivatives	3,353	4,903
Total	3,353	4,903
Level 3 [Member]
Assets:
Short-term investments
Fixed maturities - available for sale	329	330
Preferred stock
Common stock
Derivatives
Total	329	330
Liabilities:
Guaranteed residual values of TRAC leases
Derivatives
Total