Fair Value Measurements (Financial instruments level within the fair value hierarchy) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Mar. 31, 2017
Assets:
Short-term investments	$ 778,878	$ 521,911
Fixed maturities - available for sale	1,829,863	1,625,845
Preferred stock	11,986	13,489
Common stock	26,465	24,434
Derivatives	4,190	4,260
Total	2,651,382	2,189,939
Liabilities:
Guaranteed residual values of TRAC leases
Derivatives	1,253	4,903
Total	1,253	4,903
Level 1 [Member]
Assets:
Short-term investments	778,678	521,710
Fixed maturities - available for sale	7,647	6,491
Preferred stock	11,986	13,489
Common stock	26,465	24,434
Derivatives	4,190	4,260
Total	828,966	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	1,821,938	1,619,024
Preferred stock
Common stock
Derivatives
Total	1,822,138	1,619,225
Liabilities:
Guaranteed residual values of TRAC leases
Derivatives	1,253	4,903
Total	1,253	4,903
Level 3 [Member]
Assets:
Short-term investments
Fixed maturities - available for sale	278	330
Preferred stock
Common stock
Derivatives
Total	278	330
Liabilities:
Guaranteed residual values of TRAC leases
Derivatives
Total