Adjusted Cost and Estimated Market Value Available-for-sale Investments (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Mar. 31, 2012
Available-for-sale securities, amortized cost:
Amortized cost	$ 1,027,985	$ 730,025
Available-for-sale securities, fair value:
Estimated market value	1,095,338	766,792
U.S. treasury, government, and corporate securities
Available-for-sale securities, amortized cost:
Due in one year or less	43,658	40,219
Due after one year through five years	180,448	157,444
Due after five years through ten years	281,546	176,694
Due after ten years	448,169	291,990
Amortized cost	953,821	666,347
Available-for-sale securities, fair value:
Due in one year or less	44,333	40,688
Due after one year through five years	191,930	165,852
Due after five years through ten years	302,188	188,225
Due after ten years	486,780	319,160
Estimated market value	1,025,231	713,925
Mortgage-backed securities
Available-for-sale securities, amortized cost:
Amortized cost	6,730	11,572
Available-for-sale securities, fair value:
Estimated market value	6,985	11,811
Redeemable preferred stock
Available-for-sale securities, amortized cost:
Amortized cost	23,705	24,370
Available-for-sale securities, fair value:
Estimated market value	24,212	23,177
Common Stock
Available-for-sale securities, amortized cost:
Amortized cost	43,729	27,736
Available-for-sale securities, fair value:
Estimated market value	$ 38,910	$ 17,879