Investments (Securities Held to Maturity) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule of Held-to-maturity Securities [Line Items]
Total	$ 7,224,459	$ 7,285,254
Gross Unrealized Gains	314,097	57,319
Gross Unrealized Losses	(891)	(114,305)
Fair Value	7,537,665	7,228,268
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	12,086	2,833,188
Less than 12 Months, Unrealized Losses	(109)	(67,652)
12 Months or Greater, Fair Value	68,793	1,073,028
12 Months or Greater, Unrealized Losses	(782)	(46,653)
Total, Fair Value	80,879	3,906,216
Total, Unrealized Losses	(891)	(114,305)
U.S. agencies
Schedule of Held-to-maturity Securities [Line Items]
Total	105,201
Gross Unrealized Gains	1,883
Gross Unrealized Losses	0
Fair Value	107,084
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position [Abstract]
Total, Unrealized Losses	0
U.S. Treasury
Schedule of Held-to-maturity Securities [Line Items]
Total	3,777	1,341
Gross Unrealized Gains	156	116
Gross Unrealized Losses	0	0
Fair Value	3,933	1,457
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position [Abstract]
Total, Unrealized Losses	0	0
States and political subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Total	441,257	457,404
Gross Unrealized Gains	21,832	9,764
Gross Unrealized Losses	(53)	(2,376)
Fair Value	463,036	464,792
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	0	88,253
Less than 12 Months, Unrealized Losses	0	(2,124)
12 Months or Greater, Fair Value	1,689	10,645
12 Months or Greater, Unrealized Losses	(53)	(252)
Total, Fair Value	1,689	98,898
Total, Unrealized Losses	(53)	(2,376)
Foreign governments
Schedule of Held-to-maturity Securities [Line Items]
Total	1,149
Gross Unrealized Gains	56
Gross Unrealized Losses	0
Fair Value	1,205
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position [Abstract]
Total, Unrealized Losses	0
Public utilities
Schedule of Held-to-maturity Securities [Line Items]
Total	913,445	930,629
Gross Unrealized Gains	36,887	5,928
Gross Unrealized Losses	(11)	(12,944)
Fair Value	950,321	923,613
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	2,417	396,980
Less than 12 Months, Unrealized Losses	(11)	(8,371)
12 Months or Greater, Fair Value	0	98,632
12 Months or Greater, Unrealized Losses	0	(4,573)
Total, Fair Value	2,417	495,612
Total, Unrealized Losses	(11)	(12,944)
Corporate
Schedule of Held-to-maturity Securities [Line Items]
Total	4,654,880	4,715,775
Gross Unrealized Gains	211,359	27,652
Gross Unrealized Losses	(446)	(87,043)
Fair Value	4,865,793	4,656,384
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	9,220	2,144,969
Less than 12 Months, Unrealized Losses	(95)	(55,125)
12 Months or Greater, Fair Value	20,213	650,401
12 Months or Greater, Unrealized Losses	(351)	(31,918)
Total, Fair Value	29,433	2,795,370
Total, Unrealized Losses	(446)	(87,043)
Commercial mortgage-backed
Schedule of Held-to-maturity Securities [Line Items]
Total	3,039
Gross Unrealized Gains	57
Gross Unrealized Losses	0
Fair Value	3,096
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position [Abstract]
Total, Unrealized Losses	0
Residential mortgage-backed
Schedule of Held-to-maturity Securities [Line Items]
Total	1,098,612	1,176,216
Gross Unrealized Gains	41,789	13,771
Gross Unrealized Losses	(381)	(11,932)
Fair Value	1,140,020	1,178,055
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	449	202,986
Less than 12 Months, Unrealized Losses	(3)	(2,032)
12 Months or Greater, Fair Value	46,891	311,374
12 Months or Greater, Unrealized Losses	(378)	(9,900)
Total, Fair Value	47,340	514,360
Total, Unrealized Losses	(381)	(11,932)
Asset-backed
Schedule of Held-to-maturity Securities [Line Items]
Total	3,099	3,889
Gross Unrealized Gains	78	88
Gross Unrealized Losses	0	(10)
Fair Value	3,177	3,967
Debt Securities, Held-to-maturity, 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		1,976
12 Months or Greater, Unrealized Losses		(10)
Total, Fair Value		1,976
Total, Unrealized Losses	$ 0	$ (10)