Investments (Securities Held to Maturity) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 6,345,116	$ 5,962,120
Gross Unrealized Gains	326,360	546,604
Gross Unrealized Losses	(97,290)	(5,097)
Fair Value	6,574,186	6,503,627
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	1,956,395	362,671
Less than 12 Months, Unrealized Losses	(94,934)	(2,518)
12 Months or Greater, Fair Value	31,859	47,965
12 Months or Greater, Unrealized Losses	(2,356)	(2,579)
Total, Fair Value	1,988,254	410,636
Total, Unrealized Losses	(97,290)	(5,097)
U.S. Agencies [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	23,095	23,114
Gross Unrealized Gains	1,955	2,748
Gross Unrealized Losses	0	0
Fair Value	25,050	25,862
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
U.S. Treasury [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,912	1,907
Gross Unrealized Gains	492	648
Gross Unrealized Losses	0	0
Fair Value	2,404	2,555
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	420,618	391,062
Gross Unrealized Gains	19,007	41,150
Gross Unrealized Losses	(9,433)	(431)
Fair Value	430,192	431,781
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	96,058	19,745
Less than 12 Months, Unrealized Losses	(9,303)	(401)
12 Months or Greater, Fair Value	1,419	1,470
12 Months or Greater, Unrealized Losses	(130)	(30)
Total, Fair Value	97,477	21,215
Total, Unrealized Losses	(9,433)	(431)
Foreign Governments [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	9,994	9,988
Gross Unrealized Gains	279	616
Gross Unrealized Losses	0	0
Fair Value	10,273	10,604
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	800,954	781,239
Gross Unrealized Gains	60,551	89,162
Gross Unrealized Losses	(6,995)	(103)
Fair Value	854,510	870,298
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	167,547	24,271
Less than 12 Months, Unrealized Losses	(6,995)	(80)
12 Months or Greater, Fair Value	0	1,982
12 Months or Greater, Unrealized Losses	0	(23)
Total, Fair Value	167,547	26,253
Total, Unrealized Losses	(6,995)	(103)
Corporate [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	3,345,778	2,887,572
Gross Unrealized Gains	166,137	273,431
Gross Unrealized Losses	(65,345)	(3,753)
Fair Value	3,446,570	3,157,250
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	1,263,953	303,645
Less than 12 Months, Unrealized Losses	(63,154)	(1,776)
12 Months or Greater, Fair Value	27,796	38,078
12 Months or Greater, Unrealized Losses	(2,191)	(1,977)
Total, Fair Value	1,291,749	341,723
Total, Unrealized Losses	(65,345)	(3,753)
Mortgage-backed [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,714,586	1,835,051
Gross Unrealized Gains	72,554	133,684
Gross Unrealized Losses	(15,482)	(261)
Fair Value	1,771,658	1,968,474
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	428,837	15,010
Less than 12 Months, Unrealized Losses	(15,482)	(261)
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total, Fair Value	428,837	15,010
Total, Unrealized Losses	(15,482)	(261)
Home Equity [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	20,400	21,545
Gross Unrealized Gains	4,858	4,443
Gross Unrealized Losses	(35)	(549)
Fair Value	25,223	25,439
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	2,644	6,435
12 Months or Greater, Unrealized Losses	(35)	(549)
Total, Fair Value	2,644	6,435
Total, Unrealized Losses	(35)	(549)
Manufactured Housing [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	7,779	10,642
Gross Unrealized Gains	527	722
Gross Unrealized Losses	0	0
Fair Value	8,306	11,364
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