Investments (Available For Sale Investments) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Mar. 31, 2017
Available-for-sale securities, investments:
Amortized cost	$ 1,777,297	$ 1,630,600
Gross unrealized gains	75,336	55,740
Gross unrealized losses more than 12 months	(3,808)	(5,770)
Gross unrealized losses less than 12 months	(5,057)	(16,802)
Estimated market value	1,843,768	1,663,768
U.S. treasury securities and government obligations [Member]
Available-for-sale securities, investments:
Amortized cost	123,503	123,474
Gross unrealized gains	3,468	2,892
Gross unrealized losses more than 12 months
Gross unrealized losses less than 12 months	(1,176)	(1,675)
Estimated market value	125,795	124,691
U.S. government agency mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	26,812	27,908
Gross unrealized gains	1,355	1,070
Gross unrealized losses more than 12 months	(2)	(6)
Gross unrealized losses less than 12 months	(2)	(377)
Estimated market value	28,163	28,595
Obligations of states and political subdivisions [Member]
Available-for-sale securities, investments:
Amortized cost	174,665	159,417
Gross unrealized gains	10,626	9,466
Gross unrealized losses more than 12 months	(10)	(23)
Gross unrealized losses less than 12 months	(223)	(424)
Estimated market value	185,058	168,436
Corporate securities [Member]
Available-for-sale securities, investments:
Amortized cost	1,332,938	1,263,703
Gross unrealized gains	48,148	32,901
Gross unrealized losses more than 12 months	(3,786)	(5,731)
Gross unrealized losses less than 12 months	(3,631)	(13,837)
Estimated market value	1,373,669	1,277,036
Mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	89,858	26,577
Gross unrealized gains	1,958	515
Gross unrealized losses more than 12 months
Gross unrealized losses less than 12 months		(5)
Estimated market value	91,816	27,087
Redeemable preferred stocks [Member]
Available-for-sale securities, investments:
Amortized cost	13,789	13,789
Gross unrealized gains	539	168
Gross unrealized losses more than 12 months
Gross unrealized losses less than 12 months	(25)	(468)
Estimated market value	14,303	13,489
Common stocks [Member]
Available-for-sale securities, investments:
Amortized cost	15,732	15,732
Gross unrealized gains	9,242	8,728
Gross unrealized losses more than 12 months	(10)	(10)
Gross unrealized losses less than 12 months		(16)
Estimated market value	$ 24,964	$ 24,434