Investments (Securities Held to Maturity) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 7,042,233	$ 6,841,543
Gross Unrealized Gains	434,263	357,805
Gross Unrealized Losses	(6,363)	(23,905)
Fair Value	7,470,133	7,175,443
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	132,157	183,821
Less than 12 Months, Unrealized Losses	(1,244)	(3,307)
12 Months or Greater, Fair Value	428,810	951,284
12 Months or Greater, Unrealized Losses	(5,119)	(20,598)
Total, Fair Value	560,967	1,135,105
Total, Unrealized Losses	(6,363)	(23,905)
U.S. Agencies [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	15,047	10,061
Gross Unrealized Gains	634	705
Gross Unrealized Losses	0	0
Fair Value	15,681	10,766
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	1,921	1,920
Gross Unrealized Gains	414	409
Gross Unrealized Losses	0	0
Fair Value	2,335	2,329
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	436,398	432,186
Gross Unrealized Gains	35,935	31,417
Gross Unrealized Losses	(250)	(336)
Fair Value	472,083	463,267
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	13,220	23,076
12 Months or Greater, Unrealized Losses	(250)	(336)
Total, Fair Value	13,220	23,076
Total, Unrealized Losses	(250)	(336)
Foreign Governments [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	0
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	0
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Total, Unrealized Losses	0
Public Utilities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	975,817	978,847
Gross Unrealized Gains	78,915	67,836
Gross Unrealized Losses	(68)	(757)
Fair Value	1,054,664	1,045,926
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	2,003	7,078
Less than 12 Months, Unrealized Losses	(5)	(13)
12 Months or Greater, Fair Value	18,445	48,198
12 Months or Greater, Unrealized Losses	(63)	(744)
Total, Fair Value	20,448	55,276
Total, Unrealized Losses	(68)	(757)
Corporate [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	3,975,303	3,754,222
Gross Unrealized Gains	226,933	183,650
Gross Unrealized Losses	(4,642)	(18,591)
Fair Value	4,197,594	3,919,281
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	99,277	156,839
Less than 12 Months, Unrealized Losses	(1,080)	(2,997)
12 Months or Greater, Fair Value	308,553	698,316
12 Months or Greater, Unrealized Losses	(3,562)	(15,594)
Total, Fair Value	407,830	855,155
Total, Unrealized Losses	(4,642)	(18,591)
Mortgage-backed [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,614,749	1,640,582
Gross Unrealized Gains	86,462	68,726
Gross Unrealized Losses	(1,403)	(4,164)
Fair Value	1,699,808	1,705,144
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	30,877	17,698
Less than 12 Months, Unrealized Losses	(159)	(240)
12 Months or Greater, Fair Value	88,592	181,694
12 Months or Greater, Unrealized Losses	(1,244)	(3,924)
Total, Fair Value	119,469	199,392
Total, Unrealized Losses	(1,403)	(4,164)
Home Equity [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	18,704	18,886
Gross Unrealized Gains	4,614	4,734
Gross Unrealized Losses	0	(57)
Fair Value	23,318	23,563
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value		2,206
Less than 12 Months, Unrealized Losses		(57)
12 Months or Greater, Fair Value		0
12 Months or Greater, Unrealized Losses		0
Total, Fair Value		2,206
Total, Unrealized Losses	0	(57)
Manufactured Housing [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	4,294	4,839
Gross Unrealized Gains	356	328
Gross Unrealized Losses	0	0
Fair Value	4,650	5,167
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Total, Unrealized Losses	$ 0	$ 0