Securities in an Unrealized Loss Position (Securities in an Unrealized Loss Position) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Fair Value, Less than 12 Months	$ 2,765,686	$ 1,221,105
Gross Unrealized Losses, Less than 12 Months	79,012	11,595
Fair Value, 12 Months or Greater	235,472	315,818
Gross Unrealized Losses, 12 Months or Greater	24,300	23,423
Fair Value, Total	3,001,158	1,536,923
Gross Unrealized Losses, Total	103,312	35,018
U.S. government and agency
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Fair Value, Less than 12 Months	145,472	69,551
Gross Unrealized Losses, Less than 12 Months	4,462	1,660
Fair Value, 12 Months or Greater	0	0
Gross Unrealized Losses, 12 Months or Greater	0	0
Fair Value, Total	145,472	69,551
Gross Unrealized Losses, Total	4,462	1,660
State and municipal
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Fair Value, Less than 12 Months	644,312	152,694
Gross Unrealized Losses, Less than 12 Months	19,393	1,639
Fair Value, 12 Months or Greater	61,210	135,967
Gross Unrealized Losses, 12 Months or Greater	2,572	8,054
Fair Value, Total	705,522	288,661
Gross Unrealized Losses, Total	21,965	9,693
Mortgage-backed securities
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Fair Value, Less than 12 Months	498,700	484,731
Gross Unrealized Losses, Less than 12 Months	17,880	3,629
Fair Value, 12 Months or Greater	84,047	58,292
Gross Unrealized Losses, 12 Months or Greater	5,018	4,571
Fair Value, Total	582,747	543,023
Gross Unrealized Losses, Total	22,898	8,200
Corporate
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Fair Value, Less than 12 Months	1,069,096	371,781
Gross Unrealized Losses, Less than 12 Months	17,738	2,964
Fair Value, 12 Months or Greater	70,276	70,537
Gross Unrealized Losses, 12 Months or Greater	10,975	9,119
Fair Value, Total	1,139,372	442,318
Gross Unrealized Losses, Total	28,713	12,083
Foreign
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Fair Value, Less than 12 Months	344,301	95,623
Gross Unrealized Losses, Less than 12 Months	12,062	996
Fair Value, 12 Months or Greater	0	11,210
Gross Unrealized Losses, 12 Months or Greater	0	28
Fair Value, Total	344,301	106,833
Gross Unrealized Losses, Total	12,062	1,024
Debt Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Fair Value, Less than 12 Months	2,701,881	1,174,380
Gross Unrealized Losses, Less than 12 Months	71,535	10,888
Fair Value, 12 Months or Greater	215,533	276,006
Gross Unrealized Losses, 12 Months or Greater	18,565	21,772
Fair Value, Total	2,917,414	1,450,386
Gross Unrealized Losses, Total	90,100	32,660
Common stocks
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Fair Value, Less than 12 Months	36,899	46,725
Gross Unrealized Losses, Less than 12 Months	5,576	707
Fair Value, 12 Months or Greater	0	0
Gross Unrealized Losses, 12 Months or Greater	0	0
Fair Value, Total	36,899	46,725
Gross Unrealized Losses, Total	5,576	707
Preferred stocks
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Fair Value, Less than 12 Months	26,906	0
Gross Unrealized Losses, Less than 12 Months	1,901	0
Fair Value, 12 Months or Greater	19,939	39,812
Gross Unrealized Losses, 12 Months or Greater	5,735	1,651
Fair Value, Total	46,845	39,812
Gross Unrealized Losses, Total	7,636	1,651
Equity Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Fair Value, Less than 12 Months	63,805	46,725
Gross Unrealized Losses, Less than 12 Months	7,477	707
Fair Value, 12 Months or Greater	19,939	39,812
Gross Unrealized Losses, 12 Months or Greater	5,735	1,651
Fair Value, Total	83,744	86,537
Gross Unrealized Losses, Total	$ 13,212	$ 2,358