Investments (Securities Held to Maturity) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 6,898,038	$ 6,510,320
Gross Unrealized Gains	339,495	281,623
Gross Unrealized Losses	(42,092)	(135,799)
Fair Value	7,195,441	6,656,144
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	469,419	2,306,154
Less than 12 Months, Unrealized Losses	(2,146)	(121,620)
12 Months or Greater, Fair Value	1,256,602	149,558
12 Months or Greater, Unrealized Losses	(39,946)	(14,179)
Total, Fair Value	1,726,021	2,455,712
Total, Unrealized Losses	(42,092)	(135,799)
U.S. Agencies [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	23,069	23,088
Gross Unrealized Gains	913	1,700
Gross Unrealized Losses	0	0
Fair Value	23,982	24,788
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Total, Unrealized Losses	0	0
U.S. Treasury [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,918	1,913
Gross Unrealized Gains	407	434
Gross Unrealized Losses	0	0
Fair Value	2,325	2,347
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Total, Unrealized Losses	0	0
States and Political Subdivisions [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	430,243	423,286
Gross Unrealized Gains	27,854	13,433
Gross Unrealized Losses	(1,051)	(10,944)
Fair Value	457,046	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	33,440	8,080
12 Months or Greater, Unrealized Losses	(1,051)	(1,695)
Total, Fair Value	33,440	136,791
Total, Unrealized Losses	(1,051)	(10,944)
Foreign Governments [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	0	9,997
Gross Unrealized Gains	0	159
Gross Unrealized Losses	0	0
Fair Value	0	10,156
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Total, Unrealized Losses	0	0
Public Utilities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	915,761	864,324
Gross Unrealized Gains	63,842	53,222
Gross Unrealized Losses	(1,558)	(9,687)
Fair Value	978,045	907,859
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	73,039	260,982
Less than 12 Months, Unrealized Losses	(231)	(8,998)
12 Months or Greater, Fair Value	68,585	7,821
12 Months or Greater, Unrealized Losses	(1,327)	(689)
Total, Fair Value	141,624	268,803
Total, Unrealized Losses	(1,558)	(9,687)
Corporate [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	3,834,631	3,463,521
Gross Unrealized Gains	181,422	153,442
Gross Unrealized Losses	(26,026)	(81,760)
Fair Value	3,990,027	3,535,203
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	328,940	1,335,088
Less than 12 Months, Unrealized Losses	(1,478)	(71,330)
12 Months or Greater, Fair Value	845,310	117,179
12 Months or Greater, Unrealized Losses	(24,548)	(10,430)
Total, Fair Value	1,174,250	1,452,267
Total, Unrealized Losses	(26,026)	(81,760)
Mortgage-backed [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,667,528	1,696,887
Gross Unrealized Gains	59,881	54,035
Gross Unrealized Losses	(13,457)	(33,376)
Fair Value	1,713,952	1,717,546
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	67,440	581,373
Less than 12 Months, Unrealized Losses	(437)	(32,043)
12 Months or Greater, Fair Value	309,267	13,861
12 Months or Greater, Unrealized Losses	(13,020)	(1,333)
Total, Fair Value	376,707	595,234
Total, Unrealized Losses	(13,457)	(33,376)
Home Equity [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	19,348	20,179
Gross Unrealized Gains	4,841	4,738
Gross Unrealized Losses	0	(32)
Fair Value	24,189	24,885
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value		0
Less than 12 Months, Unrealized Losses		0
12 Months or Greater, Fair Value		2,617
12 Months or Greater, Unrealized Losses		(32)
Total, Fair Value		2,617
Total, Unrealized Losses	0	(32)
Manufactured Housing [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	5,540	7,125
Gross Unrealized Gains	335	460
Gross Unrealized Losses	0	0
Fair Value	5,875	7,585
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Total, Unrealized Losses	$ 0	$ 0