Debt - Schedule of Selected Information about Debt Issues (Details) - USD ($)	6 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Debt Instrument [Line Items]
Long-term debt, par value, gross	$ 1,650,000,000
Total short-term debt, par value	663,225,000
Total debt, par value	2,313,225,000
Unamortized discount & issuance costs, noncurrent	(19,604,000)
Unamortized discount & issuance costs, current	(8,619,000)
Total debt, unamortized discount & issuance costs,	(28,223,000)
Long-term debt (estimated fair value: 2024—$1,376,196; 2023—$1,491,229)	1,630,396,000	$ 1,629,559,000
Total short-term debt, book value	654,606,000	486,113,000
Total debt, book value	2,285,002,000	2,115,672,000
Fair Value	1,376,196,000
Total short-term debt, fair value	654,606,000
Total debt, fair value	2,030,802,000
FHLB borrowings
Debt Instrument [Line Items]
Total short-term debt, par value	180,000,000	0
Unamortized discount & issuance costs, current	0
Total short-term debt, book value	180,000,000	0
Total short-term debt, fair value	180,000,000
Commercial paper
Debt Instrument [Line Items]
Total short-term debt, par value	313,225,000
Unamortized discount & issuance costs, current	(8,423,000)
Total short-term debt, book value	304,802,000	316,564,000
Total short-term debt, fair value	$ 304,802,000
Senior Notes Due 2028
Debt Instrument [Line Items]
Coupon Rate	4.55%
Long-term debt, par value, gross	$ 550,000,000
Unamortized discount & issuance costs, noncurrent	(3,359,000)
Debt	546,641,000	546,283,000
Long-term debt, gross, fair value	$ 522,121,000
Senior Notes Due 2030
Debt Instrument [Line Items]
Coupon Rate	2.15%
Long-term debt, par value, gross	$ 400,000,000
Unamortized discount & issuance costs, noncurrent	(3,099,000)
Debt	396,901,000	396,670,000
Long-term debt, gross, fair value	$ 320,552,000
Senior Notes Due June 15 2032
Debt Instrument [Line Items]
Coupon Rate	4.80%
Long-term debt, par value, gross	$ 250,000,000
Unamortized discount & issuance costs, noncurrent	(3,928,000)
Debt	246,072,000	245,873,000
Long-term debt, gross, fair value	229,785,000
Senior Notes Due June 15 2032 | Consolidation Eliminations | Subsidiaries
Debt Instrument [Line Items]
Long-term debt, net, book value	$ 150,000,000
Junior Subordinated Debentures Due 2057
Debt Instrument [Line Items]
Coupon Rate	5.275%
Long-term debt, par value, gross	$ 125,000,000
Unamortized discount & issuance costs, noncurrent	(1,565,000)
Debt	123,435,000	123,427,000
Long-term debt, gross, fair value	$ 109,258,000
Junior Subordinated Debentures Due 2061
Debt Instrument [Line Items]
Coupon Rate	4.25%
Long-term debt, par value, gross	$ 325,000,000
Unamortized discount & issuance costs, noncurrent	(7,653,000)
Debt	317,347,000	317,306,000
Long-term debt, gross, fair value	$ 194,480,000
Term Loan Due November 11 2024
Debt Instrument [Line Items]
Coupon Rate	6.42%
Long-term debt, par value, gross	$ 170,000,000
Unamortized discount & issuance costs, noncurrent	(196,000)
Debt	169,804,000	$ 169,549,000
Long-term debt, gross, fair value	$ 169,804,000
Basis spread	1.35%