Fair Value of Financial Instruments (Carrying Amounts and Estimated Fair Values of Financial Instruments) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Assets:
Arbitrage trading account	$ 721,088	$ 617,649
Loans receivable	100,400	82,047
Carrying Value
Assets:
Fixed maturity securities	13,218,729	13,551,250
Equity securities	472,389	576,647
Arbitrage trading account	721,088	617,649
Loans receivable	98,088	79,684
Cash and cash equivalents	911,202	950,471
Trading account receivables from brokers and clearing organizations	119,628	189,280
Liabilities:
Due to broker	62,522	15,920
Trading account securities sold but not yet purchased	90,281	64,358
Subordinated debentures	907,117	728,218
Senior notes and other debt	1,792,055	1,769,052
Fair Value
Assets:
Fixed maturity securities	13,231,392	13,566,976
Equity securities	472,389	576,647
Arbitrage trading account	721,088	617,649
Loans receivable	100,400	82,047
Cash and cash equivalents	911,202	950,471
Trading account receivables from brokers and clearing organizations	119,628	189,280
Liabilities:
Due to broker	62,522	15,920
Trading account securities sold but not yet purchased	90,281	64,358
Subordinated debentures	929,297	769,060
Senior notes and other debt	$ 1,918,004	$ 1,945,313