Investments (Securities Held to Maturity) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Totals before allowance for credit losses	$ 6,827,139			$ 7,106,245
Gross Unrealized Gains	559,005			303,060
Gross Unrealized Losses	(7,745)			(1,602)
Fair Value	7,378,399			7,407,703
Allowance for Credit Losses	(5,086)	$ (4,940)	$ (3,300)	0
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	163,265			65,985
Less than 12 Months, Unrealized Losses	(7,520)			(517)
12 Months or Greater, Fair Value	7,574			62,401
12 Months or Greater, Unrealized Losses	(225)			(1,085)
Total, Fair Value	170,839			128,386
Total, Unrealized Losses	(7,745)			(1,602)
U.S. agencies
Schedule of Held-to-maturity Securities [Line Items]
Totals before allowance for credit losses	73,153			100,910
Gross Unrealized Gains	2,934			1,686
Gross Unrealized Losses	0			0
Fair Value	76,087			102,596
Allowance for Credit Losses	(81)
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position [Abstract]
Total, Unrealized Losses	0			0
U.S. Treasury
Schedule of Held-to-maturity Securities [Line Items]
Totals before allowance for credit losses	3,798			3,782
Gross Unrealized Gains	156			140
Gross Unrealized Losses	0			0
Fair Value	3,954			3,922
Allowance for Credit Losses	0
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]
Totals before allowance for credit losses	437,620			431,433
Gross Unrealized Gains	24,976			19,440
Gross Unrealized Losses	(124)			(84)
Fair Value	462,472			450,789
Allowance for Credit Losses	(176)
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	13,687			5,013
Less than 12 Months, Unrealized Losses	(55)			(33)
12 Months or Greater, Fair Value	1,757			1,712
12 Months or Greater, Unrealized Losses	(69)			(51)
Total, Fair Value	15,444			6,725
Total, Unrealized Losses	(124)			(84)
Foreign governments
Schedule of Held-to-maturity Securities [Line Items]
Totals before allowance for credit losses	1,128			1,144
Gross Unrealized Gains	150			55
Gross Unrealized Losses	0			0
Fair Value	1,278			1,199
Allowance for Credit Losses	0
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position [Abstract]
Total, Unrealized Losses	0			0
Public utilities
Schedule of Held-to-maturity Securities [Line Items]
Totals before allowance for credit losses	812,251			888,444
Gross Unrealized Gains	71,098			36,638
Gross Unrealized Losses	0			(83)
Fair Value	883,349			924,999
Allowance for Credit Losses	(718)
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	0			2,345
Less than 12 Months, Unrealized Losses	0			(83)
12 Months or Greater, Fair Value	0			0
12 Months or Greater, Unrealized Losses	0			0
Total, Fair Value	0			2,345
Total, Unrealized Losses	0			(83)
Corporate
Schedule of Held-to-maturity Securities [Line Items]
Totals before allowance for credit losses	4,549,262			4,607,826
Gross Unrealized Gains	401,764			212,281
Gross Unrealized Losses	(7,620)			(718)
Fair Value	4,943,406			4,819,389
Allowance for Credit Losses	(4,111)
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	148,574			31,419
Less than 12 Months, Unrealized Losses	(7,464)			(337)
12 Months or Greater, Fair Value	5,817			17,191
12 Months or Greater, Unrealized Losses	(156)			(381)
Total, Fair Value	154,391			48,610
Total, Unrealized Losses	(7,620)			(718)
Commercial mortgage-backed
Schedule of Held-to-maturity Securities [Line Items]
Totals before allowance for credit losses	3,019			3,032
Gross Unrealized Gains	34			52
Gross Unrealized Losses	0			0
Fair Value	3,053			3,084
Allowance for Credit Losses	0
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	0
12 Months or Greater, Unrealized Losses	0
Total, Fair Value	0
Total, Unrealized Losses	0			0
Residential mortgage-backed
Schedule of Held-to-maturity Securities [Line Items]
Totals before allowance for credit losses	944,838			1,066,899
Gross Unrealized Gains	57,854			32,706
Gross Unrealized Losses	(1)			(716)
Fair Value	1,002,691			1,098,889
Allowance for Credit Losses	0
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	1,004			25,859
Less than 12 Months, Unrealized Losses	(1)			(63)
12 Months or Greater, Fair Value	0			43,498
12 Months or Greater, Unrealized Losses	0			(653)
Total, Fair Value	1,004			69,357
Total, Unrealized Losses	(1)			(716)
Asset-backed
Schedule of Held-to-maturity Securities [Line Items]
Totals before allowance for credit losses	2,070			2,775
Gross Unrealized Gains	39			62
Gross Unrealized Losses	0			(1)
Fair Value	2,109			2,836
Allowance for Credit Losses	0
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	0			1,349
Less than 12 Months, Unrealized Losses	0			(1)
12 Months or Greater, Fair Value	0			0
12 Months or Greater, Unrealized Losses	0			0
Total, Fair Value	0			1,349
Total, Unrealized Losses	$ 0			$ (1)