Investments (Securities Held to Maturity) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule of Held-to-maturity Securities [Line Items]
Totals	$ 6,841,543	$ 6,510,320
Gross Unrealized Gains	357,805	281,623
Gross Unrealized Losses	(23,905)	(135,799)
Fair Value	7,175,443	6,656,144
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	183,821	2,306,154
Less than 12 Months, Unrealized Losses	(3,307)	(121,620)
12 Months or Greater, Fair Value	951,284	149,558
12 Months or Greater, Unrealized Losses	(20,598)	(14,179)
Total, Fair Value	1,135,105	2,455,712
Gross Unrealized Losses	(23,905)	(135,799)
U.S. Agencies [Member]
Schedule of Held-to-maturity Securities [Line Items]
Totals	10,061	23,088
Gross Unrealized Gains	705	1,700
Gross Unrealized Losses	0	0
Fair Value	10,766	24,788
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Gross Unrealized Losses	0	0
U.S. Treasury [Member]
Schedule of Held-to-maturity Securities [Line Items]
Totals	1,920	1,913
Gross Unrealized Gains	409	434
Gross Unrealized Losses	0	0
Fair Value	2,329	2,347
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Gross Unrealized Losses	0	0
States and Political Subdivisions [Member]
Schedule of Held-to-maturity Securities [Line Items]
Totals	432,186	423,286
Gross Unrealized Gains	31,417	13,433
Gross Unrealized Losses	(336)	(10,944)
Fair Value	463,267	425,775
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	0	128,711
Less than 12 Months, Unrealized Losses	0	(9,249)
12 Months or Greater, Fair Value	23,076	8,080
12 Months or Greater, Unrealized Losses	(336)	(1,695)
Total, Fair Value	23,076	136,791
Gross Unrealized Losses	(336)	(10,944)
Foreign Governments [Member]
Schedule of Held-to-maturity Securities [Line Items]
Totals		9,997
Gross Unrealized Gains		159
Gross Unrealized Losses		0
Fair Value		10,156
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Gross Unrealized Losses		0
Public Utilities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Totals	978,847	864,324
Gross Unrealized Gains	67,836	53,222
Gross Unrealized Losses	(757)	(9,687)
Fair Value	1,045,926	907,859
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	7,078	260,982
Less than 12 Months, Unrealized Losses	(13)	(8,998)
12 Months or Greater, Fair Value	48,198	7,821
12 Months or Greater, Unrealized Losses	(744)	(689)
Total, Fair Value	55,276	268,803
Gross Unrealized Losses	(757)	(9,687)
Corporate [Member]
Schedule of Held-to-maturity Securities [Line Items]
Totals	3,754,222	3,463,521
Gross Unrealized Gains	183,650	153,442
Gross Unrealized Losses	(18,591)	(81,760)
Fair Value	3,919,281	3,535,203
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	156,839	1,335,088
Less than 12 Months, Unrealized Losses	(2,997)	(71,330)
12 Months or Greater, Fair Value	698,316	117,179
12 Months or Greater, Unrealized Losses	(15,594)	(10,430)
Total, Fair Value	855,155	1,452,267
Gross Unrealized Losses	(18,591)	(81,760)
Mortgage-backed [Member]
Schedule of Held-to-maturity Securities [Line Items]
Totals	1,640,582	1,696,887
Gross Unrealized Gains	68,726	54,035
Gross Unrealized Losses	(4,164)	(33,376)
Fair Value	1,705,144	1,717,546
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	17,698	581,373
Less than 12 Months, Unrealized Losses	(240)	(32,043)
12 Months or Greater, Fair Value	181,694	13,861
12 Months or Greater, Unrealized Losses	(3,924)	(1,333)
Total, Fair Value	199,392	595,234
Gross Unrealized Losses	(4,164)	(33,376)
Home Equity [Member]
Schedule of Held-to-maturity Securities [Line Items]
Totals	18,886	20,179
Gross Unrealized Gains	4,734	4,738
Gross Unrealized Losses	(57)	(32)
Fair Value	23,563	24,885
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	2,206	0
Less than 12 Months, Unrealized Losses	(57)	0
12 Months or Greater, Fair Value	0	2,617
12 Months or Greater, Unrealized Losses	0	(32)
Total, Fair Value	2,206	2,617
Gross Unrealized Losses	(57)	(32)
Manufactured Housing [Member]
Schedule of Held-to-maturity Securities [Line Items]
Totals	4,839	7,125
Gross Unrealized Gains	328	460
Gross Unrealized Losses	0	0
Fair Value	5,167	7,585
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Gross Unrealized Losses	$ 0	$ 0