Fair Value of Financial Instruments (Carrying Amounts and Estimated Fair Values of Financial Instruments) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Assets:
Arbitrage trading account	$ 666,829	$ 400,809
Loans receivable	91,074	94,613
Carrying Value
Assets:
Fixed maturity securities	13,666,067	14,180,961
Equity securities	356,102	480,620
Arbitrage trading account	666,829	400,809
Loans receivable	87,058	91,799
Cash and cash equivalents	965,012	1,023,710
Trading account receivables from brokers and clearing organizations	132,928	423,543
Due from broker	11,476	0
Liabilities:
Due to broker	0	27,116
Trading account securities sold but not yet purchased	14,003	36,143
Subordinated debentures	1,198,951	1,198,704
Senior notes and other debt	1,432,117	1,427,575
Fair Value
Assets:
Fixed maturity securities	13,678,391	14,194,955
Equity securities	356,102	480,620
Arbitrage trading account	666,829	400,809
Loans receivable	91,074	94,613
Cash and cash equivalents	965,012	1,023,710
Trading account receivables from brokers and clearing organizations	132,928	423,543
Due from broker	11,476	0
Liabilities:
Due to broker	0	27,116
Trading account securities sold but not yet purchased	14,003	36,143
Subordinated debentures	1,092,780	1,274,088
Senior notes and other debt	$ 1,454,081	$ 1,582,290