Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Mar. 31, 2017
Available-for-sale securities, amortized cost:
Amortized cost	$ 1,777,297	$ 1,630,600
Available-for-sale securities, fair value:
Estimated market value	1,843,768	1,663,768
Us Treasury, Government, and Corporate Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	40,057	35,399
Due after one year through five years	366,506	324,286
Due after five years through ten years	609,614	598,232
Due after ten years	641,741	616,585
Available-for-sale securities, fair value:
Due in one year or less	40,428	35,795
Due after one year through five years	377,028	333,016
Due after five years through ten years	628,689	607,184
Due after ten years	666,540	622,763
Mortgage-backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	89,858	26,577
Available-for-sale securities, fair value:
Estimated market value	91,816	27,087
Redeemable preferred stocks [Member]
Available-for-sale securities, amortized cost:
Amortized cost	13,789	13,789
Available-for-sale securities, fair value:
Estimated market value	14,303	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	$ 24,964	$ 24,434