Investments (Securities Held to Maturity) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,962,120	$ 5,641,909
Gross Unrealized Gains	546,604	461,771
Gross Unrealized Losses	(5,097)	(21,350)
Total	6,503,627	6,082,330
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	362,671	275,489
Less than 12 Months, Unrealized Losses	(2,518)	(8,730)
12 Months or Greater, Fair Value	47,965	171,440
12 Months or Greater, Unrealized Losses	(2,579)	(12,620)
Total, Fair Value	410,636	446,929
Total, Unrealized Losses	(5,097)	(21,350)
U.S. Agencies [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	23,114	205,464
Gross Unrealized Gains	2,748	3,519
Gross Unrealized Losses	0	(330)
Total	25,862	208,653
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	0	24,985
Less than 12 Months, Unrealized Losses	0	(16)
12 Months or Greater, Fair Value	0	27,010
12 Months or Greater, Unrealized Losses	0	(314)
Total, Fair Value	0	51,995
Total, Unrealized Losses	0	(330)
U.S. Treasury [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,907	1,937
Gross Unrealized Gains	648	648
Gross Unrealized Losses	0	0
Total	2,555	2,585
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	0	0
Less than 12 Months, Unrealized Losses	0	0
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total, Fair Value	0	0
Total, Unrealized Losses	0	0
States and Political Subdivisions [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	391,062	358,364
Gross Unrealized Gains	41,150	27,338
Gross Unrealized Losses	(431)	(280)
Total	431,781	385,422
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	19,745	805
Less than 12 Months, Unrealized Losses	(401)	(82)
12 Months or Greater, Fair Value	1,470	17,117
12 Months or Greater, Unrealized Losses	(31)	(198)
Total, Fair Value	21,215	17,922
Total, Unrealized Losses	(432)	(280)
Foreign Governments [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	9,988	9,979
Gross Unrealized Gains	616	927
Gross Unrealized Losses	0	0
Total	10,604	10,906
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	0	0
Less than 12 Months, Unrealized Losses	0	0
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total, Fair Value	0	0
Total, Unrealized Losses	0	0
Public Utilities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	781,239	685,989
Gross Unrealized Gains	89,162	77,060
Gross Unrealized Losses	(103)	(4,498)
Total	870,298	758,551
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	24,271	26,509
Less than 12 Months, Unrealized Losses	(80)	(1,388)
12 Months or Greater, Fair Value	1,982	17,242
12 Months or Greater, Unrealized Losses	(23)	(3,110)
Total, Fair Value	26,253	43,751
Total, Unrealized Losses	(103)	(4,498)
Corporate [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,887,572	2,258,640
Gross Unrealized Gains	273,431	195,551
Gross Unrealized Losses	(3,753)	(14,483)
Total	3,157,250	2,439,708
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	303,645	199,934
Less than 12 Months, Unrealized Losses	(1,776)	(7,215)
12 Months or Greater, Fair Value	38,078	95,975
12 Months or Greater, Unrealized Losses	(1,977)	(7,268)
Total, Fair Value	341,723	295,909
Total, Unrealized Losses	(3,753)	(14,483)
Mortgage-backed [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,835,051	2,082,650
Gross Unrealized Gains	133,684	155,413
Gross Unrealized Losses	(261)	(29)
Total	1,968,474	2,238,034
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	15,010	23,256
Less than 12 Months, Unrealized Losses	(261)	(29)
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total, Fair Value	15,010	23,256
Total, Unrealized Losses	(261)	(29)
Home Equity [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	21,545	23,815
Gross Unrealized Gains	4,443	439
Gross Unrealized Losses	(549)	(1,649)
Total	25,439	22,605
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	0	0
Less than 12 Months, Unrealized Losses	0	0
12 Months or Greater, Fair Value	6,435	11,660
12 Months or Greater, Unrealized Losses	(548)	(1,649)
Total, Fair Value	6,435	11,660
Total, Unrealized Losses	(548)	(1,649)
Manufactured Housing [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	10,642	15,071
Gross Unrealized Gains	722	876
Gross Unrealized Losses	0	(81)
Total	11,364	15,866
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	0	0
Less than 12 Months, Unrealized Losses	0	0
12 Months or Greater, Fair Value	0	2,436
12 Months or Greater, Unrealized Losses	0	(81)
Total, Fair Value	0	2,436
Total, Unrealized Losses	$ 0	$ (81)