Securities in an Unrealized Loss Position (Securities in an Unrealized Loss Position) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	$ 3,085,977	$ 1,221,105
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	84,259	11,595
Available-for-sale Securities, 12 Months or Greater, Fair Value	355,098	315,818
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	33,194	23,423
Available-for-sale Securities, Fair Value	3,441,075	1,536,923
Available For Sale Securities, Gross Unrealized Losses	117,453	35,018
U.S. government and government agency
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	260,882	69,551
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	7,927	1,660
Available-for-sale Securities, 12 Months or Greater, Fair Value	2,163	0
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	55	0
Available-for-sale Securities, Fair Value	263,045	69,551
Available For Sale Securities, Gross Unrealized Losses	7,982	1,660
State and municipal
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	899,613	152,694
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	24,503	1,639
Available-for-sale Securities, 12 Months or Greater, Fair Value	87,345	135,967
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	5,334	8,054
Available-for-sale Securities, Fair Value	986,958	288,661
Available For Sale Securities, Gross Unrealized Losses	29,837	9,693
Mortgage-backed securities
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	578,603	484,731
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	17,964	3,629
Available-for-sale Securities, 12 Months or Greater, Fair Value	140,648	58,292
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	8,965	4,571
Available-for-sale Securities, Fair Value	719,251	543,023
Available For Sale Securities, Gross Unrealized Losses	26,929	8,200
Corporate
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	1,013,373	398,595
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	17,066	3,406
Available-for-sale Securities, 12 Months or Greater, Fair Value	105,074	70,537
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	13,034	9,119
Available-for-sale Securities, Fair Value	1,118,447	469,132
Available For Sale Securities, Gross Unrealized Losses	30,100	12,525
Foreign government
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	320,215	68,809
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	16,286	554
Available-for-sale Securities, 12 Months or Greater, Fair Value	0	11,210
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	0	28
Available-for-sale Securities, Fair Value	320,215	80,019
Available For Sale Securities, Gross Unrealized Losses	16,286	582
Fixed maturity securities
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	3,072,686	1,174,380
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	83,746	10,888
Available-for-sale Securities, 12 Months or Greater, Fair Value	335,230	276,006
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	27,388	21,772
Available-for-sale Securities, Fair Value	3,407,916	1,450,386
Available For Sale Securities, Gross Unrealized Losses	111,134	32,660
Common stocks
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	0	46,725
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	0	707
Available-for-sale Securities, 12 Months or Greater, Fair Value	0	0
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	0	0
Available-for-sale Securities, Fair Value	0	46,725
Available For Sale Securities, Gross Unrealized Losses	0	707
Preferred stocks
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	13,291	0
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	513	0
Available-for-sale Securities, 12 Months or Greater, Fair Value	19,868	39,812
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	5,806	1,651
Available-for-sale Securities, Fair Value	33,159	39,812
Available For Sale Securities, Gross Unrealized Losses	6,319	1,651
Equity securities available for sale
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Less Than 12 Months, Fair Value	13,291	46,725
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	513	707
Available-for-sale Securities, 12 Months or Greater, Fair Value	19,868	39,812
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	5,806	1,651
Available-for-sale Securities, Fair Value	33,159	86,537
Available For Sale Securities, Gross Unrealized Losses	$ 6,319	$ 2,358