Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Mar. 31, 2016
Available-for-sale securities, amortized cost:
Amortized cost	$ 1,603,186	$ 1,496,256
Available-for-sale securities, fair value:
Estimated market value	1,656,159	1,510,538
US Treasury Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	31,772	48,679
Due after one year through five years	298,190	250,576
Due after five years through ten years	628,082	557,984
Due after ten years	568,583	560,317
Amortized cost	1,526,627	1,417,556
Available-for-sale securities, fair value:
Due in one year or less	32,158	49,146
Due after one year through five years	307,633	256,597
Due after five years through ten years	644,932	557,961
Due after ten years	585,035	559,833
Estimated market value	1,569,758	1,423,537
Mortgage-backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	42,850	42,991
Available-for-sale securities, fair value:
Estimated market value	43,698	43,404
Redeemable preferred stocks [Member]
Available-for-sale securities, amortized cost:
Amortized cost	15,977	17,977
Available-for-sale securities, fair value:
Estimated market value	16,316	18,428
Common stocks [Member]
Available-for-sale securities, amortized cost:
Amortized cost	17,732	17,732
Available-for-sale securities, fair value:
Estimated market value	$ 26,387	$ 25,169