Investments (Available For Sale Investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Mar. 31, 2017
Available-for-sale securities, investments:
Amortized cost	$ 1,715,968	$ 1,630,600
Gross unrealized gains	62,734	55,740
Gross unrealized losses more than 12 months	(5,357)	(5,770)
Gross unrealized losses less than 12 months	(10,314)	(16,802)
Estimated market value	1,763,031	1,663,768
U.S. treasury securities and government obligations [Member]
Available-for-sale securities, investments:
Amortized cost	123,328	123,474
Gross unrealized gains	3,169	2,892
Gross unrealized losses more than 12 months
Gross unrealized losses less than 12 months	(1,605)	(1,675)
Estimated market value	124,892	124,691
U.S. government agency mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	27,330	27,908
Gross unrealized gains	1,005	1,070
Gross unrealized losses more than 12 months	(6)	(6)
Gross unrealized losses less than 12 months	(327)	(377)
Estimated market value	28,002	28,595
Obligations of states and political subdivisions [Member]
Available-for-sale securities, investments:
Amortized cost	165,559	159,417
Gross unrealized gains	9,917	9,466
Gross unrealized losses more than 12 months	(19)	(23)
Gross unrealized losses less than 12 months	(308)	(424)
Estimated market value	175,149	168,436
Corporate securities [Member]
Available-for-sale securities, investments:
Amortized cost	1,289,221	1,263,703
Gross unrealized gains	38,855	32,901
Gross unrealized losses more than 12 months	(5,322)	(5,731)
Gross unrealized losses less than 12 months	(7,744)	(13,837)
Estimated market value	1,315,010	1,277,036
Mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	81,009	26,577
Gross unrealized gains	550	515
Gross unrealized losses more than 12 months
Gross unrealized losses less than 12 months	(282)	(5)
Estimated market value	81,277	27,087
Redeemable preferred stocks [Member]
Available-for-sale securities, investments:
Amortized cost	13,789	13,789
Gross unrealized gains	348	168
Gross unrealized losses more than 12 months
Gross unrealized losses less than 12 months	(46)	(468)
Estimated market value	14,091	13,489
Common stocks [Member]
Available-for-sale securities, investments:
Amortized cost	15,732	15,732
Gross unrealized gains	8,890	8,728
Gross unrealized losses more than 12 months	(10)	(10)
Gross unrealized losses less than 12 months	(2)	(16)
Estimated market value	$ 24,610	$ 24,434