Investments (Securities Held to Maturity) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Total	$ 7,230,187	$ 7,159,259
Gross Unrealized Gains	250,736	220,495
Gross Unrealized Losses	(14,783)	(42,143)
Fair Value	7,466,140	7,337,611
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	775,212	1,580,333
Less than 12 Months, Unrealized Losses	(12,147)	(34,830)
12 Months or Greater, Fair Value	155,117	188,056
12 Months or Greater, Unrealized Losses	(2,636)	(7,313)
Total, Fair Value	930,329	1,768,389
Total, Unrealized Losses	(14,783)	(42,143)
U.S. Treasury
Schedule of Held-to-maturity Securities [Line Items]
Total	1,335	1,333
Gross Unrealized Gains	218	235
Gross Unrealized Losses	0	0
Fair Value	1,553	1,568
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Total, Unrealized Losses	0	0
States and political subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Total	449,964	456,069
Gross Unrealized Gains	23,622	22,697
Gross Unrealized Losses	(790)	(2,841)
Fair Value	472,796	475,925
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	31,453	80,507
Less than 12 Months, Unrealized Losses	(790)	(2,841)
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total, Fair Value	31,453	80,507
Total, Unrealized Losses	(790)	(2,841)
Public utilities
Schedule of Held-to-maturity Securities [Line Items]
Total	1,114,289	1,087,176
Gross Unrealized Gains	37,840	36,904
Gross Unrealized Losses	(716)	(3,133)
Fair Value	1,151,413	1,120,947
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	71,504	162,587
Less than 12 Months, Unrealized Losses	(716)	(3,133)
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total, Fair Value	71,504	162,587
Total, Unrealized Losses	(716)	(3,133)
Corporate
Schedule of Held-to-maturity Securities [Line Items]
Total	4,327,597	4,237,029
Gross Unrealized Gains	145,011	116,720
Gross Unrealized Losses	(9,862)	(29,701)
Fair Value	4,462,746	4,324,048
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	481,789	1,063,194
Less than 12 Months, Unrealized Losses	(7,580)	(22,867)
12 Months or Greater, Fair Value	146,064	179,113
12 Months or Greater, Unrealized Losses	(2,282)	(6,834)
Total, Fair Value	627,853	1,242,307
Total, Unrealized Losses	(9,862)	(29,701)
Residential mortgage-backed
Schedule of Held-to-maturity Securities [Line Items]
Total	1,327,836	1,367,270
Gross Unrealized Gains	42,565	42,345
Gross Unrealized Losses	(3,415)	(6,468)
Fair Value	1,366,986	1,403,147
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	190,466	274,045
Less than 12 Months, Unrealized Losses	(3,061)	(5,989)
12 Months or Greater, Fair Value	9,053	8,943
12 Months or Greater, Unrealized Losses	(354)	(479)
Total, Fair Value	199,519	282,988
Total, Unrealized Losses	(3,415)	(6,468)
Home equity
Schedule of Held-to-maturity Securities [Line Items]
Total	7,881	8,826
Gross Unrealized Gains	1,376	1,462
Gross Unrealized Losses	0	0
Fair Value	9,257	10,288
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Total, Unrealized Losses	0	0
Manufactured housing
Schedule of Held-to-maturity Securities [Line Items]
Total	1,285	1,556
Gross Unrealized Gains	104	132
Gross Unrealized Losses	0	0
Fair Value	1,389	1,688
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Total, Unrealized Losses	$ 0	$ 0