Available-for-Sale Investments (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Mar. 31, 2013
Available-for-sale securities, investments:
Amortized cost	$ 1,097,704
Gross unrealized gains	45,613
Gross unrealized losses more than 12 months	(2,865)
Gross unrealized losses less than 12 months	(26,112)
Estimated market value	1,114,340	1,095,338
U.S. treasury, government, and corporate securities
Available-for-sale securities, investments:
Amortized cost	1,036,295
Gross unrealized gains	42,880
Gross unrealized losses more than 12 months	(1,475)
Gross unrealized losses less than 12 months	(25,692)
Estimated market value	1,052,008
U.S. treasury securities and government obligation
Available-for-sale securities, investments:
Amortized cost	28,048
Gross unrealized gains	1,932
Gross unrealized losses more than 12 months	(3)
Gross unrealized losses less than 12 months	(293)
Estimated market value	29,684
U.S. government agency mortgage-backed securities
Available-for-sale securities, investments:
Amortized cost	44,735
Gross unrealized gains	2,772
Gross unrealized losses more than 12 months	(4)
Gross unrealized losses less than 12 months	(1,009)
Estimated market value	46,494
Obligations of states and political subdivisions
Available-for-sale securities, investments:
Amortized cost	162,678
Gross unrealized gains	9,304
Gross unrealized losses more than 12 months	(132)
Gross unrealized losses less than 12 months	(1,916)
Estimated market value	169,934
Corporate securities
Available-for-sale securities, investments:
Amortized cost	800,834
Gross unrealized gains	28,872
Gross unrealized losses more than 12 months	(1,336)
Gross unrealized losses less than 12 months	(22,474)
Estimated market value	805,896
Mortgage-backed securities
Available-for-sale securities, investments:
Amortized cost	5,613
Gross unrealized gains	177
Gross unrealized losses more than 12 months	(35)
Gross unrealized losses less than 12 months	0
Estimated market value	5,755
Redeemable preferred stock
Available-for-sale securities, investments:
Amortized cost	18,440
Gross unrealized gains	385
Gross unrealized losses more than 12 months	(64)
Gross unrealized losses less than 12 months	(312)
Estimated market value	18,449
Common stock
Available-for-sale securities, investments:
Amortized cost	37,356
Gross unrealized gains	2,171
Gross unrealized losses more than 12 months	(1,291)
Gross unrealized losses less than 12 months	(108)
Estimated market value	$ 38,128