Investments (Securities Held to Maturity) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Totals before allowance for credit losses	$ 6,903,782			$ 7,106,245
Gross Unrealized Gains	536,045			303,060
Gross Unrealized Losses	(7,125)			(1,602)
Fair Value	7,432,702			7,407,703
Allowance for Credit Losses	(4,940)	$ (6,039)	$ (3,300)	0
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	157,752			65,985
Less than 12 Months, Unrealized Losses	(7,068)			(517)
12 Months or Greater, Fair Value	1,749			62,401
12 Months or Greater, Unrealized Losses	(57)			(1,085)
Total, Fair Value	159,501			128,386
Total, Unrealized Losses	(7,125)			(1,602)
U.S. agencies
Schedule of Held-to-maturity Securities [Line Items]
Totals before allowance for credit losses	81,855			100,910
Gross Unrealized Gains	3,501			1,686
Gross Unrealized Losses	0			0
Fair Value	85,356			102,596
Allowance for Credit Losses	(4)
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	3,792			3,782
Gross Unrealized Gains	190			140
Gross Unrealized Losses	0			0
Fair Value	3,982			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	401,264			431,433
Gross Unrealized Gains	23,915			19,440
Gross Unrealized Losses	(57)			(84)
Fair Value	425,122			450,789
Allowance for Credit Losses	(65)
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	0			5,013
Less than 12 Months, Unrealized Losses	0			(33)
12 Months or Greater, Fair Value	1,749			1,712
12 Months or Greater, Unrealized Losses	(57)			(51)
Total, Fair Value	1,749			6,725
Total, Unrealized Losses	(57)			(84)
Foreign governments
Schedule of Held-to-maturity Securities [Line Items]
Totals before allowance for credit losses	1,133			1,144
Gross Unrealized Gains	154			55
Gross Unrealized Losses	0			0
Fair Value	1,287			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	854,139			888,444
Gross Unrealized Gains	64,981			36,638
Gross Unrealized Losses	0			(83)
Fair Value	919,120			924,999
Allowance for Credit Losses	(879)
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	4,559,466			4,607,826
Gross Unrealized Gains	378,165			212,281
Gross Unrealized Losses	(7,068)			(718)
Fair Value	4,930,563			4,819,389
Allowance for Credit Losses	(3,992)
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	157,752			31,419
Less than 12 Months, Unrealized Losses	(7,068)			(337)
12 Months or Greater, Fair Value	0			17,191
12 Months or Greater, Unrealized Losses	0			(381)
Total, Fair Value	157,752			48,610
Total, Unrealized Losses	(7,068)			(718)
Commercial mortgage-backed
Schedule of Held-to-maturity Securities [Line Items]
Totals before allowance for credit losses	3,024			3,032
Gross Unrealized Gains	52			52
Gross Unrealized Losses	0			0
Fair Value	3,076			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	996,802			1,066,899
Gross Unrealized Gains	65,045			32,706
Gross Unrealized Losses	0			(716)
Fair Value	1,061,847			1,098,889
Allowance for Credit Losses	0
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position [Abstract]
Less than 12 Months, Fair Value	0			25,859
Less than 12 Months, Unrealized Losses	0			(63)
12 Months or Greater, Fair Value	0			43,498
12 Months or Greater, Unrealized Losses	0			(653)
Total, Fair Value	0			69,357
Total, Unrealized Losses	0			(716)
Asset-backed
Schedule of Held-to-maturity Securities [Line Items]
Totals before allowance for credit losses	2,307			2,775
Gross Unrealized Gains	42			62
Gross Unrealized Losses	0			(1)
Fair Value	2,349			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)