Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Statement of Financial Position [Abstract]
Fixed income securities, amortized cost	$ 6,121,910	$ 5,927,254
Fixed income securities pledged as collateral, amortized cost	125,090	126,196
Short-term investments, amortized cost	$ 215,103	$ 271,118
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	45,003,512	45,003,461
Common stock, shares outstanding	45,003,512	45,003,461
Treasury stock, shares	937	937