Investments (Securities Held to Maturity) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 6,078,141	$ 5,962,120
Gross Unrealized Gains	518,150	546,604
Gross Unrealized Losses	(7,417)	(5,097)
Fair Value	6,588,874	6,503,627
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	457,269	362,671
Less than 12 Months, Unrealized Losses	(5,301)	(2,518)
12 Months or Greater, Fair Value	35,200	47,965
12 Months or Greater, Unrealized Losses	(2,116)	(2,579)
Total, Fair Value	492,469	410,636
Total, Unrealized Losses	(7,417)	(5,097)
U.S. Agencies [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	23,107	23,114
Gross Unrealized Gains	2,551	2,748
Gross Unrealized Losses	0	0
Fair Value	25,658	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	1,909	1,907
Gross Unrealized Gains	617	648
Gross Unrealized Losses	0	0
Fair Value	2,526	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	415,553	391,062
Gross Unrealized Gains	40,932	41,150
Gross Unrealized Losses	(718)	(431)
Fair Value	455,767	431,781
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	21,683	19,745
Less than 12 Months, Unrealized Losses	(712)	(401)
12 Months or Greater, Fair Value	579	1,470
12 Months or Greater, Unrealized Losses	(6)	(31)
Total, Fair Value	22,262	21,215
Total, Unrealized Losses	(718)	(432)
Foreign Governments [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	9,990	9,988
Gross Unrealized Gains	507	616
Gross Unrealized Losses	0	0
Fair Value	10,497	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	778,321	781,239
Gross Unrealized Gains	86,375	89,162
Gross Unrealized Losses	(181)	(103)
Fair Value	864,515	870,298
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	23,292	24,271
Less than 12 Months, Unrealized Losses	(181)	(80)
12 Months or Greater, Fair Value	0	1,982
12 Months or Greater, Unrealized Losses	0	(23)
Total, Fair Value	23,292	26,253
Total, Unrealized Losses	(181)	(103)
Corporate [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	3,052,451	2,887,572
Gross Unrealized Gains	260,064	273,431
Gross Unrealized Losses	(5,675)	(3,753)
Fair Value	3,306,840	3,157,250
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	381,377	303,645
Less than 12 Months, Unrealized Losses	(3,924)	(1,776)
12 Months or Greater, Fair Value	28,235	38,078
12 Months or Greater, Unrealized Losses	(1,751)	(1,977)
Total, Fair Value	409,612	341,723
Total, Unrealized Losses	(5,675)	(3,753)
Mortgage-backed [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,766,021	1,835,051
Gross Unrealized Gains	121,769	133,684
Gross Unrealized Losses	(484)	(261)
Fair Value	1,887,306	1,968,474
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	30,917	15,010
Less than 12 Months, Unrealized Losses	(484)	(261)
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total, Fair Value	30,917	15,010
Total, Unrealized Losses	(484)	(261)
Home Equity [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	21,057	21,545
Gross Unrealized Gains	4,556	4,443
Gross Unrealized Losses	(359)	(549)
Fair Value	25,254	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	6,386	6,435
12 Months or Greater, Unrealized Losses	(359)	(548)
Total, Fair Value	6,386	6,435
Total, Unrealized Losses	(359)	(548)
Manufactured Housing [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	9,732	10,642
Gross Unrealized Gains	779	722
Gross Unrealized Losses	0	0
Fair Value	10,511	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