Adjusted Cost and Estimated Market Value Available-for-sale Investments (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Mar. 31, 2013
Available-for-sale securities, amortized cost:
Amortized cost	$ 1,097,704
Available-for-sale securities, fair value:
Estimated market value	1,114,340	1,095,338
U.S. treasury, government, and corporate securities
Available-for-sale securities, amortized cost:
Due in one year or less	25,259
Due after one year through five years	186,894
Due after five years through ten years	316,233
Due after ten years	507,909
Amortized cost	1,036,295
Available-for-sale securities, fair value:
Due in one year or less	25,520
Due after one year through five years	195,909
Due after five years through ten years	320,354
Due after ten years	510,225
Estimated market value	1,052,008
Mortgage-backed securities
Available-for-sale securities, amortized cost:
Amortized cost	5,613
Available-for-sale securities, fair value:
Estimated market value	5,755
Redeemable preferred stock
Available-for-sale securities, amortized cost:
Amortized cost	18,440
Available-for-sale securities, fair value:
Estimated market value	18,449
Common stock
Available-for-sale securities, amortized cost:
Amortized cost	37,356
Available-for-sale securities, fair value:
Estimated market value	$ 38,128