Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Statement of Financial Position [Abstract]
Fixed income securities, amortized cost	$ 4,813,186	$ 4,514,878
Fixed income securities pledged as collateral, amortized cost	34,732	64,378
Short-term investments, amortized cost	197,070	360,069
Other investments	$ 331,697	$ 240,969
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	20,000,000	20,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	130,000,000	130,000,000
Common stock, shares issued	45,006,139	45,005,932
Common stock, shares outstanding	45,006,139	45,005,932
Treasury stock, shares	2,459	2,459