Investments (Securities Held to Maturity) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities [Line Items]
Total	$ 7,280,200	$ 7,247,024
Gross Unrealized Gains	97,560	202,170
Gross Unrealized Losses	(75,726)	(15,090)
Fair Value	7,302,034	7,434,104
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	2,698,321	456,365
Less than 12 Months, Unrealized Losses	(46,676)	(2,459)
12 Months or Greater, Fair Value	646,356	662,950
12 Months or Greater, Unrealized Losses	(29,050)	(12,631)
Total, Fair Value	3,344,677	1,119,315
Total, Unrealized Losses	(75,726)	(15,090)
U.S. Treasury
Schedule of Held-to-maturity Securities [Line Items]
Total	1,338	1,337
Gross Unrealized Gains	150	177
Gross Unrealized Losses	0	0
Fair Value	1,488	1,514
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	462,708	467,437
Gross Unrealized Gains	14,097	21,907
Gross Unrealized Losses	(1,580)	(100)
Fair Value	475,225	489,244
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	64,701	6,308
Less than 12 Months, Unrealized Losses	(1,393)	(14)
12 Months or Greater, Fair Value	4,768	4,869
12 Months or Greater, Unrealized Losses	(187)	(86)
Total, Fair Value	69,469	11,177
Total, Unrealized Losses	(1,580)	(100)
Public utilities
Schedule of Held-to-maturity Securities [Line Items]
Total	1,038,226	1,062,545
Gross Unrealized Gains	14,664	30,527
Gross Unrealized Losses	(8,408)	(894)
Fair Value	1,044,482	1,092,178
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	408,005	68,368
Less than 12 Months, Unrealized Losses	(7,032)	(407)
12 Months or Greater, Fair Value	33,205	34,091
12 Months or Greater, Unrealized Losses	(1,376)	(487)
Total, Fair Value	441,210	102,459
Total, Unrealized Losses	(8,408)	(894)
Corporate
Schedule of Held-to-maturity Securities [Line Items]
Total	4,521,036	4,430,099
Gross Unrealized Gains	52,589	121,978
Gross Unrealized Losses	(49,835)	(7,876)
Fair Value	4,523,790	4,544,201
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	1,833,024	248,844
Less than 12 Months, Unrealized Losses	(32,380)	(1,296)
12 Months or Greater, Fair Value	420,683	431,591
12 Months or Greater, Unrealized Losses	(17,455)	(6,580)
Total, Fair Value	2,253,707	680,435
Total, Unrealized Losses	(49,835)	(7,876)
Residential mortgage-backed
Schedule of Held-to-maturity Securities [Line Items]
Total	1,251,971	1,280,307
Gross Unrealized Gains	15,936	27,445
Gross Unrealized Losses	(15,895)	(6,216)
Fair Value	1,252,012	1,301,536
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	389,982	130,015
Less than 12 Months, Unrealized Losses	(5,863)	(738)
12 Months or Greater, Fair Value	187,700	192,399
12 Months or Greater, Unrealized Losses	(10,032)	(5,478)
Total, Fair Value	577,682	322,414
Total, Unrealized Losses	(15,895)	(6,216)
Home equity
Schedule of Held-to-maturity Securities [Line Items]
Total	3,994	4,262
Gross Unrealized Gains	56	57
Gross Unrealized Losses	(8)	(4)
Fair Value	4,042	4,315
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	2,609	2,830
Less than 12 Months, Unrealized Losses	(8)	(4)
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total, Fair Value	2,609	2,830
Total, Unrealized Losses	(8)	(4)
Manufactured housing
Schedule of Held-to-maturity Securities [Line Items]
Total	927	1,037
Gross Unrealized Gains	68	79
Gross Unrealized Losses	0	0
Fair Value	995	1,116
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Total, Unrealized Losses	$ 0	$ 0