Investments (Securities Held to Maturity) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule of Held-to-maturity Securities [Line Items]
Totals	$ 7,173,967	$ 6,841,543
Gross Unrealized Gains	247,892	357,805
Gross Unrealized Losses	(86,423)	(23,905)
Fair Value	7,335,436	7,175,443
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	1,986,981	183,821
Less than 12 Months, Unrealized Losses	(63,361)	(3,307)
12 Months or Greater, Fair Value	445,573	951,284
12 Months or Greater, Unrealized Losses	(23,062)	(20,598)
Total, Fair Value	2,432,554	1,135,105
Gross Unrealized Losses	(86,423)	(23,905)
U.S. Agencies [Member]
Schedule of Held-to-maturity Securities [Line Items]
Totals	15,019	10,061
Gross Unrealized Gains	275	705
Gross Unrealized Losses	0	0
Fair Value	15,294	10,766
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	1,927	1,920
Gross Unrealized Gains	317	409
Gross Unrealized Losses	0	0
Fair Value	2,244	2,329
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	435,941	432,186
Gross Unrealized Gains	29,129	31,417
Gross Unrealized Losses	(662)	(336)
Fair Value	464,408	463,267
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	16,763	0
Less than 12 Months, Unrealized Losses	(387)	0
12 Months or Greater, Fair Value	8,723	23,076
12 Months or Greater, Unrealized Losses	(275)	(336)
Total, Fair Value	25,486	23,076
Gross Unrealized Losses	(662)	(336)
Public Utilities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Totals	1,044,063	978,847
Gross Unrealized Gains	42,271	67,836
Gross Unrealized Losses	(6,621)	(757)
Fair Value	1,079,713	1,045,926
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	298,962	7,078
Less than 12 Months, Unrealized Losses	(5,953)	(13)
12 Months or Greater, Fair Value	17,840	48,198
12 Months or Greater, Unrealized Losses	(668)	(744)
Total, Fair Value	316,802	55,276
Gross Unrealized Losses	(6,621)	(757)
Corporate [Member]
Schedule of Held-to-maturity Securities [Line Items]
Totals	4,160,628	3,754,222
Gross Unrealized Gains	114,920	183,650
Gross Unrealized Losses	(72,913)	(18,591)
Fair Value	4,202,635	3,919,281
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	1,522,544	156,839
Less than 12 Months, Unrealized Losses	(54,295)	(2,997)
12 Months or Greater, Fair Value	323,567	698,316
12 Months or Greater, Unrealized Losses	(18,618)	(15,594)
Total, Fair Value	1,846,111	855,155
Gross Unrealized Losses	(72,913)	(18,591)
Mortgage-backed [Member]
Schedule of Held-to-maturity Securities [Line Items]
Totals	1,503,021	1,640,582
Gross Unrealized Gains	59,013	68,726
Gross Unrealized Losses	(6,227)	(4,164)
Fair Value	1,555,807	1,705,144
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	148,712	17,698
Less than 12 Months, Unrealized Losses	(2,726)	(240)
12 Months or Greater, Fair Value	95,443	181,694
12 Months or Greater, Unrealized Losses	(3,501)	(3,924)
Total, Fair Value	244,155	199,392
Gross Unrealized Losses	(6,227)	(4,164)
Home Equity [Member]
Schedule of Held-to-maturity Securities [Line Items]
Totals	11,047	18,886
Gross Unrealized Gains	1,701	4,734
Gross Unrealized Losses	0	(57)
Fair Value	12,748	23,563
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	0	2,206
Less than 12 Months, Unrealized Losses	0	(57)
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total, Fair Value	0	2,206
Gross Unrealized Losses	0	(57)
Manufactured Housing [Member]
Schedule of Held-to-maturity Securities [Line Items]
Totals	2,321	4,839
Gross Unrealized Gains	266	328
Gross Unrealized Losses	0	0
Fair Value	2,587	5,167
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Gross Unrealized Losses	$ 0	$ 0