Investments (Available For Sale Investments) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Mar. 31, 2017
Available-for-sale securities, investments:
Amortized cost	$ 1,795,935	$ 1,630,600
Gross unrealized gains	79,428	55,740
Gross unrealized losses more than 12 months	(3,886)	(5,770)
Gross unrealized losses less than 12 months	(3,163)	(16,802)
Estimated market value	1,868,314	1,663,768
U.S. treasury securities and government obligations [Member]
Available-for-sale securities, investments:
Amortized cost	123,695	123,474
Gross unrealized gains	3,529	2,892
Gross unrealized losses more than 12 months	(770)
Gross unrealized losses less than 12 months	(390)	(1,675)
Estimated market value	126,064	124,691
U.S. government agency mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	26,598	27,908
Gross unrealized gains	1,373	1,070
Gross unrealized losses more than 12 months	(1)	(6)
Gross unrealized losses less than 12 months	(5)	(377)
Estimated market value	27,965	28,595
Obligations of states and political subdivisions [Member]
Available-for-sale securities, investments:
Amortized cost	173,039	159,417
Gross unrealized gains	10,055	9,466
Gross unrealized losses more than 12 months	(148)	(23)
Gross unrealized losses less than 12 months	(81)	(424)
Estimated market value	182,865	168,436
Corporate securities [Member]
Available-for-sale securities, investments:
Amortized cost	1,351,074	1,263,703
Gross unrealized gains	51,228	32,901
Gross unrealized losses more than 12 months	(2,957)	(5,731)
Gross unrealized losses less than 12 months	(2,556)	(13,837)
Estimated market value	1,396,789	1,277,036
Mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	94,208	26,577
Gross unrealized gains	2,072	515
Gross unrealized losses more than 12 months
Gross unrealized losses less than 12 months	(100)	(5)
Estimated market value	96,180	27,087
Redeemable preferred stocks [Member]
Available-for-sale securities, investments:
Amortized cost	11,589	13,789
Gross unrealized gains	428	168
Gross unrealized losses more than 12 months
Gross unrealized losses less than 12 months	(31)	(468)
Estimated market value	11,986	13,489
Common stocks [Member]
Available-for-sale securities, investments:
Amortized cost	15,732	15,732
Gross unrealized gains	10,743	8,728
Gross unrealized losses more than 12 months	(10)	(10)
Gross unrealized losses less than 12 months		(16)
Estimated market value	$ 26,465	$ 24,434