Available-for-Sale Investments (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Mar. 31, 2013
Available-for-sale securities, investments:
Amortized cost	$ 1,108,511
Gross unrealized gains	43,650
Gross unrealized losses more than 12 months	(575)
Gross unrealized losses less than 12 months	(29,465)
Estimated market value	1,122,121	1,095,338
U.S. treasury, government, and corporate securities
Available-for-sale securities, investments:
Amortized cost	1,066,425
Gross unrealized gains	40,304
Gross unrealized losses more than 12 months	(64)
Gross unrealized losses less than 12 months	(28,429)
Estimated market value	1,078,236
U.S. treasury securities and government obligation
Available-for-sale securities, investments:
Amortized cost	34,222
Gross unrealized gains	1,877
Gross unrealized losses more than 12 months	(2)
Gross unrealized losses less than 12 months	(544)
Estimated market value	35,553
U.S. government agency mortgage-backed securities
Available-for-sale securities, investments:
Amortized cost	42,401
Gross unrealized gains	2,585
Gross unrealized losses more than 12 months	(5)
Gross unrealized losses less than 12 months	(818)
Estimated market value	44,163
Obligations of states and political subdivisions
Available-for-sale securities, investments:
Amortized cost	164,451
Gross unrealized gains	7,287
Gross unrealized losses more than 12 months	0
Gross unrealized losses less than 12 months	(2,793)
Estimated market value	168,945
Corporate securities
Available-for-sale securities, investments:
Amortized cost	825,351
Gross unrealized gains	28,555
Gross unrealized losses more than 12 months	(57)
Gross unrealized losses less than 12 months	(24,274)
Estimated market value	829,575
Mortgage-backed securities
Available-for-sale securities, investments:
Amortized cost	5,161
Gross unrealized gains	321
Gross unrealized losses more than 12 months	0
Gross unrealized losses less than 12 months	(40)
Estimated market value	5,442
Redeemable preferred stock
Available-for-sale securities, investments:
Amortized cost	18,440
Gross unrealized gains	274
Gross unrealized losses more than 12 months	(95)
Gross unrealized losses less than 12 months	(924)
Estimated market value	17,695
Common stock
Available-for-sale securities, investments:
Amortized cost	18,485
Gross unrealized gains	2,751
Gross unrealized losses more than 12 months	(416)
Gross unrealized losses less than 12 months	(72)
Estimated market value	$ 20,748