Fair Value of Financial Instruments (Carrying Amounts and Estimated Fair Values of Financial Instruments) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Assets
Fixed maturity securities	$ 11,943,956	$ 11,312,037
Fixed maturity securities, fair value	11,968,376	11,330,910
Equity securities available for sale	376,022	443,439
Equity securities available for sale, fair value	376,022	443,439
Arbitrage trading account	329,077	397,312
Arbitrage trading account, fair value	329,077	397,312
Loans receivable	401,961	263,187	263,187
Loans receivable, fair value	406,443	245,169
Cash and cash equivalents	905,670	911,742	642,952	515,430
Cash and cash equivalents, fair value	905,670	911,742
Trading account receivable from brokers and clearing organizations	446,873	318,240
Trading account receivables from brokers and clearing organizations, fair value	446,873	318,240
Due from broker	14,449	10,875
Due from broker fair value	14,449	10,875
Liabilities
Trading account securities sold but not yet purchased	121,487	62,514
Trading account securities sold but not yet purchased, fair value	121,487	62,514
Junior subordinated debentures	243,206	242,997
Junior subordinated debentures, fair value	252,000	258,400
Senior notes and other debt	1,871,535	1,500,503
Senior notes and other debt, fair value	$ 2,190,173	$ 1,587,473

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Amount of equity securities categorized neither as held-to-maturity nor as trading.

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