Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Mar. 31, 2016
Available-for-sale securities, amortized cost:
Amortized cost	$ 1,539,357	$ 1,496,256
Available-for-sale securities, fair value:
Estimated market value	1,632,420	1,510,538
Us Treasury, Government, and Corporate Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	29,344	48,679
Due after one year through five years	316,614	250,576
Due after five years through ten years	570,836	557,984
Due after ten years	546,496	560,317
Amortized cost	1,463,290	1,417,556
Available-for-sale securities, fair value:
Due in one year or less	29,790	49,146
Due after one year through five years	330,006	256,597
Due after five years through ten years	602,486	557,961
Due after ten years	582,748	559,833
Estimated market value	1,545,030	1,423,537
Mortgage-backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	42,765	42,991
Available-for-sale securities, fair value:
Estimated market value	43,965	43,404
Redeemable preferred stocks [Member]
Available-for-sale securities, amortized cost:
Amortized cost	15,332	17,977
Available-for-sale securities, fair value:
Estimated market value	16,000	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	$ 27,425	$ 25,169