Fair Value Measurements (Financial instruments level within the fair value hierarchy) (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Mar. 31, 2023
Assets:
Short-term investments	$ 1,808,130	$ 1,809,441
Fixed maturities - available for sale	2,472,128	2,709,037
Preferred stock	19,991	21,982
Common stock	42,045	39,375
Derivatives	22,854	9,606
Total	4,365,148	4,589,441
Liabilities:
Guaranteed residual values of TRAC leases	9,630
Total	9,630
Level 1 [Member]
Assets:
Short-term investments	1,807,142	1,808,797
Fixed maturities - available for sale	168,287	251,832
Preferred stock	19,991	21,982
Common stock	42,045	39,375
Derivatives	10,526	4,295
Total	2,047,991	2,126,281
Liabilities:
Guaranteed residual values of TRAC leases	0
Total	0
Level 2 [Member]
Assets:
Short-term investments	988	644
Fixed maturities - available for sale	2,303,782	2,457,146
Preferred stock	0	0
Common stock	0	0
Derivatives	12,328	5,311
Total	2,317,098	2,463,101
Liabilities:
Guaranteed residual values of TRAC leases	0
Total	0
Level 3 [Member]
Assets:
Short-term investments	0	0
Fixed maturities - available for sale	59	59
Preferred stock	0	0
Common stock	0	0
Derivatives	0	0
Total	59	$ 59
Liabilities:
Guaranteed residual values of TRAC leases	9,630
Total	$ 9,630