Fair Value of Financial Instruments (Carrying Amounts and Estimated Fair Values of Financial Instruments) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Assets:
Arbitrage trading account	$ 504,633	$ 452,548
Loans receivable	97,791	97,073
Carrying Value
Assets:
Fixed maturity securities	13,640,669	13,606,812
Equity securities	341,981	279,006
Arbitrage trading account	504,633	452,548
Loans receivable	95,557	94,813
Cash and cash equivalents	937,428	817,602
Trading account receivables from brokers and clearing organizations	322,925	347,228
Due from broker	1,868	0
Liabilities:
Due to broker	0	20,144
Trading account securities sold but not yet purchased	58,148	38,120
Subordinated debentures	907,679	907,491
Senior notes and other debt	1,875,018	1,882,028
Fair Value
Assets:
Fixed maturity securities	13,654,337	13,619,620
Equity securities	341,981	279,006
Arbitrage trading account	504,633	452,548
Loans receivable	97,791	97,073
Cash and cash equivalents	937,428	817,602
Trading account receivables from brokers and clearing organizations	322,925	347,228
Due from broker	1,868	0
Liabilities:
Due to broker	0	20,144
Trading account securities sold but not yet purchased	58,148	38,120
Subordinated debentures	945,746	840,002
Senior notes and other debt	$ 1,985,431	$ 1,968,996