Adjusted Cost and Estimated Market Value Available-for-sale Investments (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Available-for-sale securities, amortized cost:
Amortized cost	$ 1,237,386	$ 1,135,544
Available-for-sale securities, fair value:
Estimated market value	1,304,962us-gaap_MarketableSecurities	1,138,275us-gaap_MarketableSecurities
US Government and Corporate Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	36,355	20,235
Due after one year through five years	198,488	185,447
Due after five years through ten years	474,639	350,048
Due after ten years	472,003	531,645
Amortized cost	1,181,485	1,087,375
Available-for-sale securities, fair value:
Due in one year or less	37,055	20,475
Due after one year through five years	209,404	194,563
Due after five years through ten years	492,782	350,953
Due after ten years	502,092	521,289
Estimated market value	1,241,333	1,087,280
Mortgage backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	19,874	12,425
Available-for-sale securities, fair value:
Estimated market value	20,679	12,187
Redeemable Preferred Stocks [Member]
Available-for-sale securities, amortized cost:
Amortized cost	18,052	18,445
Available-for-sale securities, fair value:
Estimated market value	18,296	17,533
Equity Securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	17,975	17,299
Available-for-sale securities, fair value:
Estimated market value	$ 24,654	$ 21,275

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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Amount after adjustments of available-for-sale debt securities at cost, maturing in the sixth fiscal year through the tenth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of available-for-sale debt securities maturing in the sixth fiscal year through the tenth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount after adjustments of available-for-sale debt securities at cost, maturing in the second fiscal year through the fifth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of available-for-sale debt securities maturing in the second fiscal year through the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount after adjustments of available-for-sale debt securities at cost, maturing after the tenth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of available-for-sale debt securities maturing after the tenth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of available-for-sale debt securities maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Total debt and equity financial instruments including: (1) securities held-to-maturity, (2) trading securities, and (3) securities available-for-sale.

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