Investments (Securities Held to Maturity) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,849,916	$ 5,641,909
Gross Unrealized Gains	569,941	461,771
Gross Unrealized Losses	(4,998)	(21,350)
Fair Value	6,414,859	6,082,330
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	162,362	275,489
Less than 12 Months, Unrealized Losses	(776)	(8,730)
12 Months or Greater, Fair Value	67,715	171,440
12 Months or Greater, Unrealized Losses	(4,222)	(12,620)
Total, Fair Value	230,077	446,929
Total, Unrealized Losses	(4,998)	(21,350)
U.S. Agencies [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	48,120	205,464
Gross Unrealized Gains	3,029	3,519
Gross Unrealized Losses	0	(330)
Fair Value	51,149	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	1,906	1,937
Gross Unrealized Gains	680	648
Gross Unrealized Losses	0	0
Fair Value	2,586	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	390,745	358,364
Gross Unrealized Gains	42,014	27,338
Gross Unrealized Losses	(112)	(280)
Fair Value	432,647	385,422
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	7,791	805
Less than 12 Months, Unrealized Losses	(113)	(82)
12 Months or Greater, Fair Value	0	17,117
12 Months or Greater, Unrealized Losses	0	(198)
Total, Fair Value	7,791	17,922
Total, Unrealized Losses	(113)	(280)
Foreign Governments [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	9,985	9,979
Gross Unrealized Gains	729	927
Gross Unrealized Losses	0	0
Fair Value	10,714	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	743,038	685,989
Gross Unrealized Gains	88,341	77,060
Gross Unrealized Losses	(212)	(4,498)
Fair Value	831,167	758,551
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	15,973	26,509
Less than 12 Months, Unrealized Losses	(11)	(1,388)
12 Months or Greater, Fair Value	13,702	17,242
12 Months or Greater, Unrealized Losses	(201)	(3,110)
Total, Fair Value	29,675	43,751
Total, Unrealized Losses	(212)	(4,498)
Corporate [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,710,117	2,258,640
Gross Unrealized Gains	280,836	195,551
Gross Unrealized Losses	(4,081)	(14,483)
Fair Value	2,986,872	2,439,708
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	136,726	199,934
Less than 12 Months, Unrealized Losses	(521)	(7,215)
12 Months or Greater, Fair Value	41,603	95,975
12 Months or Greater, Unrealized Losses	(3,559)	(7,268)
Total, Fair Value	178,329	295,909
Total, Unrealized Losses	(4,080)	(14,483)
Mortgage-backed [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,911,975	2,082,650
Gross Unrealized Gains	149,410	155,413
Gross Unrealized Losses	0	(29)
Fair Value	2,061,385	2,238,034
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	0	23,256
Less than 12 Months, Unrealized Losses	0	(29)
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total, Fair Value	0	23,256
Total, Unrealized Losses	0	(29)
Home Equity [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	21,953	23,815
Gross Unrealized Gains	4,119	439
Gross Unrealized Losses	(590)	(1,649)
Fair Value	25,482	22,605
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	1,047	0
Less than 12 Months, Unrealized Losses	(128)	0
12 Months or Greater, Fair Value	12,410	11,660
12 Months or Greater, Unrealized Losses	(462)	(1,649)
Total, Fair Value	13,457	11,660
Total, Unrealized Losses	(590)	(1,649)
Manufactured Housing [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	12,077	15,071
Gross Unrealized Gains	783	876
Gross Unrealized Losses	(3)	(81)
Fair Value	12,857	15,866
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	825	0
Less than 12 Months, Unrealized Losses	(3)	0
12 Months or Greater, Fair Value	0	2,436
12 Months or Greater, Unrealized Losses	0	(81)
Total, Fair Value	825	2,436
Total, Unrealized Losses	$ (3)	$ (81)