Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Mar. 31, 2016
Available-for-sale securities, amortized cost:
Amortized cost	$ 1,601,188	$ 1,496,256
Available-for-sale securities, fair value:
Estimated market value	1,700,581	1,510,538
Us Treasury, Government, and Corporate Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	29,111	48,679
Due after one year through five years	319,439	250,576
Due after five years through ten years	604,645	557,984
Due after ten years	572,718	560,317
Amortized cost	1,525,913	1,417,556
Available-for-sale securities, fair value:
Due in one year or less	29,500	49,146
Due after one year through five years	334,064	256,597
Due after five years through ten years	639,749	557,961
Due after ten years	612,190	559,833
Estimated market value	1,615,503	1,423,537
Mortgage-backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	42,665	42,991
Available-for-sale securities, fair value:
Estimated market value	43,821	43,404
Redeemable preferred stocks [Member]
Available-for-sale securities, amortized cost:
Amortized cost	14,640	17,977
Available-for-sale securities, fair value:
Estimated market value	15,036	18,428
Common stocks [Member]
Available-for-sale securities, amortized cost:
Amortized cost	17,970	17,732
Available-for-sale securities, fair value:
Estimated market value	$ 26,221	$ 25,169