Investments (Narratives) (Details) - USD ($) $ in Thousands	6 Months Ended
	Sep. 30, 2018	Mar. 31, 2018
Marketable Securities [Abstract]
Equity investments	$ 2,015,402	$ 1,919,860
Regulatory capital requirements
Assets Held by Insurance Regulators	31,100	32,400
Investments, fixed maturities and marketable equities
Fair value of sold available-for-sale securities	58,300
Available-for-sale securities, gross realized gains	900
Available-for-sale securities, gross realized losses	100
Preferred stocks [Member]
Marketable Securities [Abstract]
Equity investments	8,000	8,600
Common stocks [Member]
Marketable Securities [Abstract]
Equity investments	$ 26,700	$ 27,900