Fair Value of Financial Instruments (Carrying Amounts and Estimated Fair Values of Financial Instruments) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets:
Arbitrage trading account	$ 678,321	$ 617,649
Loans receivable	98,878	82,047
Carrying Value
Assets:
Fixed maturity securities	13,572,402	13,551,250
Equity securities	325,254	576,647
Arbitrage trading account	678,321	617,649
Loans receivable	96,590	79,684
Cash and cash equivalents	819,366	950,471
Trading account receivables from brokers and clearing organizations	191,394	189,280
Liabilities:
Due to broker	21,478	15,920
Trading account securities sold but not yet purchased	112,355	64,358
Subordinated debentures	907,304	728,218
Senior notes and other debt	1,790,498	1,769,052
Fair Value
Assets:
Fixed maturity securities	13,584,654	13,566,976
Equity securities	325,254	576,647
Arbitrage trading account	678,321	617,649
Loans receivable	98,878	82,047
Cash and cash equivalents	819,366	950,471
Trading account receivables from brokers and clearing organizations	191,394	189,280
Liabilities:
Due to broker	21,478	15,920
Trading account securities sold but not yet purchased	112,355	64,358
Subordinated debentures	913,344	769,060
Senior notes and other debt	$ 1,876,579	$ 1,945,313