Debt (Schedule of Debt) (Details) - USD ($)	1 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Sep. 30, 2023	Feb. 28, 2023	Jun. 30, 2021	Nov. 30, 2020	Aug. 31, 2019
Debt Instrument [Line Items]
Carrying Value		$ 2,080,600,000	$ 2,129,900,000
Interest expense		108,000,000	108,300,000	$ 111,800,000
Accrued interest		$ 33,500,000	32,300,000
Senior Notes
Debt Instrument [Line Items]
Principal Amount	$ 900,000,000
Senior Notes | 4.20% Senior Notes due September 2023
Debt Instrument [Line Items]
State interest rate	4.20%	4.20%
Principal Amount	$ 300,000,000	$ 0	225,000,000.0		$ 50,000,000
Carrying Value		$ 0	224,700,000
Senior Notes | 6.10% Senior Notes due February 2026
Debt Instrument [Line Items]
State interest rate		6.10%				6.10%
Principal Amount		$ 175,000,000.0	0			$ 175,000,000
Carrying Value		$ 173,700,000	0
Senior Notes | 4.90% Senior Notes due March 2028
Debt Instrument [Line Items]
State interest rate	4.90%	4.90%
Principal Amount	$ 300,000,000	$ 300,000,000.0	300,000,000.0
Carrying Value		$ 298,200,000	297,800,000
Senior Notes | 3.70% Senior Notes due February 2030
Debt Instrument [Line Items]
State interest rate		3.70%							3.70%
Principal Amount		$ 350,000,000.0	350,000,000.0						$ 350,000,000
Carrying Value		$ 347,900,000	347,600,000
Senior Notes | 2.65% Senior Notes due January 2032
Debt Instrument [Line Items]
State interest rate		2.65%					2.65%
Principal Amount		$ 350,000,000.0	350,000,000.0				$ 350,000,000
Carrying Value		$ 347,000,000.0	346,700,000
Senior Notes | 6.75% Senior Notes due February 2034
Debt Instrument [Line Items]
State interest rate		6.75%
Principal Amount		$ 275,000,000.0	275,000,000.0
Carrying Value		$ 272,700,000	272,500,000
Senior Notes | 5.25% Subordinated Notes due January 2061
Debt Instrument [Line Items]
State interest rate								5.25%
Principal Amount								$ 250,000,000
Subordinated Notes | 7.00% Fixed-to-Floating Rate Subordinated Notes due March 2048
Debt Instrument [Line Items]
State interest rate	7.00%	7.00%
Principal Amount	$ 400,000,000	$ 400,000,000.0	400,000,000.0
Carrying Value		$ 397,000,000.0	396,500,000
Subordinated Notes | 5.25% Subordinated Notes due January 2061
Debt Instrument [Line Items]
State interest rate		5.25%
Principal Amount		$ 250,000,000.0	250,000,000.0
Carrying Value		$ 244,100,000	$ 244,100,000
London Interbank Offered Rate (LIBOR) | Subordinated Notes | 7.00% Fixed-to-Floating Rate Subordinated Notes due March 2048
Debt Instrument [Line Items]
Basis spread on variable rate	4.135%	4.135%