Fair Value of Financial Instruments (Carrying Amounts and Estimated Fair Values of Financial Instruments) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Assets:
Arbitrage trading account	$ 744,859	$ 617,649
Loans receivable	78,238	82,047
Carrying Value
Assets:
Fixed maturity securities	13,343,132	13,551,250
Equity securities	496,034	576,647
Arbitrage trading account	744,859	617,649
Loans receivable	75,902	79,684
Cash and cash equivalents	956,603	950,471
Trading account receivables from brokers and clearing organizations	26,667	189,280
Liabilities:
Due to broker	74,450	15,920
Trading account securities sold but not yet purchased	29,453	64,358
Subordinated debentures	897,426	728,218
Senior notes and other debt	1,782,139	1,769,052
Fair Value
Assets:
Fixed maturity securities	13,356,305	13,566,976
Equity securities	496,034	576,647
Arbitrage trading account	744,859	617,649
Loans receivable	78,238	82,047
Cash and cash equivalents	956,603	950,471
Trading account receivables from brokers and clearing organizations	26,667	189,280
Liabilities:
Due to broker	74,450	15,920
Trading account securities sold but not yet purchased	29,453	64,358
Subordinated debentures	929,948	769,060
Senior notes and other debt	$ 1,921,635	$ 1,945,313