Adjusted Cost and Estimated Market Value Available-for-sale Investments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Mar. 31, 2013
Available-for-sale securities, amortized cost:
Amortized cost	$ 1,050,222
Available-for-sale securities, fair value:
Estimated market value	1,117,008	1,095,338
U.S. treasury, government, and corporate securities
Available-for-sale securities, amortized cost:
Due in one year or less	40,195
Due after one year through five years	172,665
Due after five years through ten years	293,659
Due after ten years	473,204
Amortized cost	979,723
Available-for-sale securities, fair value:
Due in one year or less	40,825
Due after one year through five years	183,996
Due after five years through ten years	315,929
Due after ten years	506,488
Estimated market value	1,047,238
Mortgage-backed securities
Available-for-sale securities, amortized cost:
Amortized cost	6,117
Available-for-sale securities, fair value:
Estimated market value	6,340
Redeemable preferred stock
Available-for-sale securities, amortized cost:
Amortized cost	21,163
Available-for-sale securities, fair value:
Estimated market value	22,000
Common stock
Available-for-sale securities, amortized cost:
Amortized cost	43,219
Available-for-sale securities, fair value:
Estimated market value	$ 41,430