Adjusted Cost and Estimated Market Value Available-for-sale Investments (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Mar. 31, 2013
Available-for-sale securities, amortized cost:
Amortized cost	$ 1,108,511
Available-for-sale securities, fair value:
Estimated market value	1,122,121	1,095,338
U.S. treasury, government, and corporate securities
Available-for-sale securities, amortized cost:
Due in one year or less	20,299
Due after one year through five years	185,073
Due after five years through ten years	338,942
Due after ten years	522,111
Amortized cost	1,066,425
Available-for-sale securities, fair value:
Due in one year or less	20,458
Due after one year through five years	194,815
Due after five years through ten years	343,111
Due after ten years	519,852
Estimated market value	1,078,236
Mortgage-backed securities
Available-for-sale securities, amortized cost:
Amortized cost	5,161
Available-for-sale securities, fair value:
Estimated market value	5,442
Redeemable preferred stock
Available-for-sale securities, amortized cost:
Amortized cost	18,440
Available-for-sale securities, fair value:
Estimated market value	17,695
Common stock
Available-for-sale securities, amortized cost:
Amortized cost	18,485
Available-for-sale securities, fair value:
Estimated market value	$ 20,748