Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Debt Securities, at cost	$ 2,178,066	$ 2,257,188
Equity Investments, fair value, cost	474,306	425,942
Other investments, portion carried at fair value	$ 33,020	$ 52,301
Common shares, par value (USD per share)	$ 0.01	$ 0.01
Common shares, shares authorized (in shares)	100,000,000	100,000,000
Common shares, shares issued (in shares)	62,986,474	62,824,523
Deferred tax expense (benefit) on accumulated other comprehensive income (loss)	$ (4,027)	$ 5,218
Treasury shares, number of shares (in shares)	9,367,545	9,367,502