Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Mar. 31, 2017
Available-for-sale securities, amortized cost:
Amortized cost	$ 1,715,968	$ 1,630,600
Available-for-sale securities, fair value:
Marketable Securities	1,763,031	1,663,768
US Treasury Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	33,197	35,399
Due after one year through five years	355,226	324,286
Due after five years through ten years	608,698	598,232
Due after ten years	608,317	616,585
Amortized cost by contractural maturity	1,605,438	1,574,502
Available-for-sale securities, fair value:
Due in one year or less	33,448	35,795
Due after one year through five years	365,085	333,016
Due after five years through ten years	622,859	607,184
Due after ten years	621,661	622,763
Estimated market value by contractural maturity	1,643,053	1,598,758
Mortgage-backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	81,009	26,577
Available-for-sale securities, fair value:
Marketable Securities	81,277	27,087
Redeemable preferred stocks [Member]
Available-for-sale securities, amortized cost:
Amortized cost	13,789	13,789
Available-for-sale securities, fair value:
Marketable Securities	14,091	13,489
Common stocks [Member]
Available-for-sale securities, amortized cost:
Amortized cost	15,732	15,732
Available-for-sale securities, fair value:
Marketable Securities	$ 24,610	$ 24,434