Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Mar. 31, 2017
Available-for-sale securities, amortized cost:
Amortized cost	$ 1,795,935	$ 1,630,600
Available-for-sale securities, fair value:
Estimated market value	1,868,314	1,663,768
Us Treasury, Government, and Corporate Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	41,025	35,399
Due after one year through five years	396,559	324,286
Due after five years through ten years	611,026	598,232
Due after ten years	625,796	616,585
Amortized cost	1,674,406	1,574,502
Available-for-sale securities, fair value:
Due in one year or less	41,353	35,795
Due after one year through five years	408,603	333,016
Due after five years through ten years	631,009	607,184
Due after ten years	652,718	622,763
Estimated market value	1,733,683	1,598,758
Mortgage-backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	94,208	26,577
Available-for-sale securities, fair value:
Estimated market value	96,180	27,087
Redeemable preferred stocks [Member]
Available-for-sale securities, amortized cost:
Amortized cost	11,589	13,789
Available-for-sale securities, fair value:
Estimated market value	11,986	13,489
Common stocks [Member]
Available-for-sale securities, amortized cost:
Amortized cost	15,732	15,732
Available-for-sale securities, fair value:
Estimated market value	$ 26,465	$ 24,434