Investments (Securities Held to Maturity) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 6,605,854	$ 6,510,320
Gross Unrealized Gains	320,869	281,623
Gross Unrealized Losses	(73,459)	(135,799)
Fair Value	6,853,264	6,656,144
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	1,538,768	2,306,154
Less than 12 Months, Unrealized Losses	(51,029)	(121,620)
12 Months or Greater, Fair Value	378,994	149,558
12 Months or Greater, Unrealized Losses	(22,430)	(14,179)
Total, Fair Value	1,917,762	2,455,712
Total, Unrealized Losses	(73,459)	(135,799)
U.S. Agencies [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	23,082	23,088
Gross Unrealized Gains	1,426	1,700
Gross Unrealized Losses	0	0
Fair Value	24,508	24,788
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	1,915	1,913
Gross Unrealized Gains	433	434
Gross Unrealized Losses	0	0
Fair Value	2,348	2,347
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	425,741	423,286
Gross Unrealized Gains	21,586	13,433
Gross Unrealized Losses	(5,059)	(10,944)
Fair Value	442,268	425,775
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	40,723	128,711
Less than 12 Months, Unrealized Losses	(2,744)	(9,249)
12 Months or Greater, Fair Value	19,738	8,080
12 Months or Greater, Unrealized Losses	(2,315)	(1,695)
Total, Fair Value	60,461	136,791
Total, Unrealized Losses	(5,059)	(10,944)
Foreign Governments [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	9,999	9,997
Gross Unrealized Gains	43	159
Gross Unrealized Losses	0	0
Fair Value	10,042	10,156
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	866,304	864,324
Gross Unrealized Gains	58,534	53,222
Gross Unrealized Losses	(3,912)	(9,687)
Fair Value	920,926	907,859
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	158,771	260,982
Less than 12 Months, Unrealized Losses	(2,696)	(8,998)
12 Months or Greater, Fair Value	22,260	7,821
12 Months or Greater, Unrealized Losses	(1,216)	(689)
Total, Fair Value	181,031	268,803
Total, Unrealized Losses	(3,912)	(9,687)
Corporate [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	3,535,559	3,463,521
Gross Unrealized Gains	173,952	153,442
Gross Unrealized Losses	(44,891)	(81,760)
Fair Value	3,664,620	3,535,203
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	850,901	1,335,088
Less than 12 Months, Unrealized Losses	(28,017)	(71,330)
12 Months or Greater, Fair Value	305,147	117,179
12 Months or Greater, Unrealized Losses	(16,874)	(10,430)
Total, Fair Value	1,156,048	1,452,267
Total, Unrealized Losses	(44,891)	(81,760)
Mortgage-backed [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,716,534	1,696,887
Gross Unrealized Gains	59,658	54,035
Gross Unrealized Losses	(19,568)	(33,376)
Fair Value	1,756,624	1,717,546
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	488,373	581,373
Less than 12 Months, Unrealized Losses	(17,572)	(32,043)
12 Months or Greater, Fair Value	29,282	13,861
12 Months or Greater, Unrealized Losses	(1,996)	(1,333)
Total, Fair Value	517,655	595,234
Total, Unrealized Losses	(19,568)	(33,376)
Home Equity [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	19,953	20,179
Gross Unrealized Gains	4,834	4,738
Gross Unrealized Losses	(29)	(32)
Fair Value	24,758	24,885
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	2,567	2,617
12 Months or Greater, Unrealized Losses	(29)	(32)
Total, Fair Value	2,567	2,617
Total, Unrealized Losses	(29)	(32)
Manufactured Housing [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	6,767	7,125
Gross Unrealized Gains	403	460
Gross Unrealized Losses	0	0
Fair Value	7,170	7,585
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